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  • CLASSES

    First generation (sedating) Antihistamines

    DEA CLASS

    Rx, OTC

    DESCRIPTION

    Oral sedating first-generation antihistamine with a longer duration of action (12 hours)
    Used to temporarily reduce symptoms of the common cold, seasonal allergies, and other respiratory allergies such as rhinorrhea, itchy, watery eyes, sneezing, and nasal and throat pruritus

    COMMON BRAND NAMES

    Dailyhist-1, Tavist Allergy

    HOW SUPPLIED

    Clemastine/Clemastine Fumarate/Dailyhist-1/Tavist Allergy Oral Tab: 1.34mg, 2.68mg

    DOSAGE & INDICATIONS

    For the treatment of symptoms associated with the common cold or allergic rhinitis including sneezing, rhinorrhea, itching of the nose or throat, and itchy, watery eyes (ocular pruritus).
    Oral dosage (non-prescription product, e.g., clemastine fumarate 1.34 mg tablets)
    Adults, Adolescents, and Children 12 years and older

    1.34 mg (1 tablet) PO every 12 hours. Max: 2 tablets in 24 hours unless under the direction of a doctor.

    Oral dosage (prescription products: 2.68 mg clemastine fumarate tablets or clemastine 0.5 mg/5mL oral syrup)
    Adults

    Initially, clemastine fumarate 1.34 mg (1 mg clemastine) PO twice daily. May increase to clemastine fumarate 2.68 mg (2 mg clemastine) PO twice daily. Do not exceed 8.04 mg/day clemastine fumarate (1 tablet PO 3 times daily or 60 mL/day oral syrup in divided doses, equivalent to 6 mg clemastine/day).

    Children and Adolescents 12 years and older

    Initially, clemastine fumarate 1.34 mg (1 mg clemastine) PO twice daily. May increase to clemastine fumarate 2.68 mg (2 mg clemastine) PO twice daily. Do not exceed 8.04 mg/day clemastine fumarate (1 tablet PO 3 times daily or 60 mL/day oral syrup in divided doses, equivalent to 6 mg clemastine/day).

    Children 6 to 11 years

    Use oral syrup for dosing. Initially, 0.5 mg (5 mL) clemastine PO twice daily. Since single doses of up to 2.25 mg clemastine were well tolerated by this age group, dosage may be increased as required. Max: 30 mL/day (3 mg/day clemastine) in divided doses.

    For the relief of mild uncomplicated allergic skin manifestations (e.g., pruritus) of urticaria and angioedema.
    Oral dosage (prescription products: 2.68 mg clemastine fumarate tablets or clemastine 0.5 mg/5mL oral syrup)
    Adults

    Initially, clemastine fumarate 2.68 mg (2 mg clemastine) PO twice daily. May increase to clemastine fumarate 2.68 mg (2 mg clemastine) PO three times daily. Max: 8.04 mg/day clemastine fumarate (1 tablet PO 3 times daily or 60 mL/day oral syrup in divided doses, equivalent to 6 mg clemastine/day).

    Children and Adolescents 12 years and older

    Initially, clemastine fumarate 2.68 mg (2 mg clemastine) PO twice daily. May increase to clemastine fumarate 2.68 mg (2 mg clemastine) PO three times daily. Max: 8.04 mg/day clemastine fumarate (1 tablet PO 3 times daily or 60 mL/day oral syrup in divided doses, equivalent to 6 mg clemastine/day).

    Children 6 to 11 years

    Use oral syrup for dosing. Initially, 1 mg (10 mL) clemastine PO twice daily. May increase to 1 mg (10 mL) clemastine PO 3 times per day if needed. Max: 30 mL/day (3 mg/day clemastine) in divided doses.

    MAXIMUM DOSAGE

    Adults

    8.04 mg/day PO clemastine fumarate; 6 mg/day PO clemastine base.

    Geriatric

    8.04 mg/day PO clemastine fumarate; 6 mg/day PO clemastine base. The elderly are more sensitive to anticholinergic effects.

    Adolescents

    8.04 mg/day PO clemastine fumarate; 6 mg/day PO clemastine base.

    Children

    12 years: 8.04 mg/day PO clemastine fumarate; 6 mg/day PO clemastine base.
    6 to 11 years: 4.02 mg/day PO clemastine fumarate; 3 mg/day PO clemastine base.
    Less than 6 years: Safety and efficacy have not been established.

    Infants

    Safety and efficacy have not been established.

    Neonates

    Use is contraindicated.

    DOSING CONSIDERATIONS

    Hepatic Impairment

    Dosage reduction may be warranted for patients with hepatic impairment; clemastine is extensively metabolized in the liver. The use of an antihistamine with a shorter half-life may preferable in patients with hepatic impairment.

    Renal Impairment

    Specific guidelines for dosage adjustments in renal impairment are not available; it appears that no dosage adjustments are needed.

    ADMINISTRATION

    Oral Administration

    Clemastine may be administered without regard to meals.

    Oral Liquid Formulations

    Oral solutions: Use a calibrated spoon or other measuring device to measure dosage.

    STORAGE

    Generic:
    - Store at controlled room temperature (between 68 and 77 degrees F)
    Dailyhist-1:
    - Store at room temperature (between 59 to 86 degrees F)
    Tavist:
    - Protect from freezing
    - Protect from light
    - Store at controlled room temperature (between 68 and 77 degrees F)
    Tavist Allergy:
    - Store at room temperature (between 59 to 86 degrees F)

    CONTRAINDICATIONS / PRECAUTIONS

    Asthma, chronic obstructive pulmonary disease (COPD)

    Use antihistamines with caution in patients with asthma. The anticholinergic activity of H1-antagonists may result in thickened bronchial secretions in the respiratory tract thereby aggravating an acute asthmatic attack or chronic obstructive pulmonary disease (COPD). Although H1-antagonists should be avoided during an acute asthmatic attack, these anticholinergic effects do not preclude the use of H1-antagonists in all asthmatic or COPD patients, particularly if the above respiratory symptom is not a primary component of the illness. Clemastine exhibits a substantial amount of anticholinergic activity, and the use of an H1-antagonist with less anticholinergic effects may be preferable in some instances.

    Children, infants, neonates

    Clemastine is contraindicated for use in neonates and premature infants and generally should not be used in infants of any age due to the increased risk for CNS stimulation in these populations. Use with caution in children of all ages since CNS stimulation can occur. There have been a number of cases of respiratory depression, sleep apnea, and SIDS in children receiving phenothiazine antihistamines. The mechanism of this reaction is not yet known; therefore, antihistamines should be used with extreme caution in children with a family history of SIDS or sleep apnea. In January 2007, the CDC warned caregivers and healthcare providers of the risk for serious injury or fatal overdose from the administration of cough and cold products to children and infants less than 2 years of age. This warning followed an investigation of the deaths of three (3) infants less than 6 months of age that were attributed to the inadvertent inappropriate use of these products. The symptoms preceding these deaths have not been clearly defined, and there is a lack of conclusive data describing the exact cause of death. The report estimated that 1519 children less than 2 years of age were treated in emergency departments during 2004—2005 for adverse events related to cough and cold medications. In October 2007, the FDA Nonprescription Drug Advisory Committee and the Pediatric Advisory Committee recommended that nonprescription cough and cold products containing pseudoephedrine, dextromethorphan, chlorpheniramine, diphenhydramine, brompheniramine, phenylephrine, clemastine, or guaifenesin not be used in children less than 6 years of age. In January 2008, the FDA issued a Public Health Advisory recommending that OTC cough and cold products not be used in infants and children less than 2 years. An official ruling regarding the use of these products in children greater than 2 years has not yet been announced. The FDA recommends that if parents and caregivers use cough and cold products in children greater than 2 years, labels should be read carefully, caution should be used when administering multiple products, and only measuring devices specifically designed for use with medications should be used. While some combination cough/cold products containing these ingredients are available by prescription only and are not necessarily under scrutiny by the FDA, clinicians should thoroughly assess each patient's use of similar products, both prescription and nonprescription, to avoid duplication of therapy and the potential for inadvertent overdose.

    Pregnancy

    Clemastine is classified as FDA pregnancy risk category B. Oral reproduction studies performed with clemastine fumarate in rats and rabbits at doses up to 312 and 188 times the adult human doses respectively, have revealed no evidence of teratogenic effects; however adequate human data are lacking. Most antihistamines generally are not recommended for use in pregnancy, especially during the third trimester, because of a seizure risk to the fetus. Additionally, antihistamines should be avoided in the last 2 weeks of pregnancy due to a possible association between these drugs and retrolental fibroplasia in premature neonates. Other antihistamines would be preferred to clemastine if an antihistamine is necessary for use. If an antihistamine is required in pregnancy, use of an agent with more data (e.g., loratadine) in human pregnancy is recommended by the British Society for Allergy and Clinical Immunology. Use should be limited to the lowest dose, and limited to short-term, 'as needed' administration under the prescription of a qualified health-care professional. Non-pharmacologic methods (e.g., fluids, rest) are recommended to be tried first for symptomatic relief of colds or allergies during pregnancy.

    Breast-feeding

    According to the manufacturer, clemastine is contraindicated for use during breast-feeding because antihistamines can induce hyperexcitability in the infant and seizures in premature infants. In one telephone follow-up study, mothers reported irritability and colicky symptoms in 10% of infants exposed to various antihistamines and drowsiness was reported in 1.6% of infants. No reactions were reported as requiring medical attention. Antihistamines can lower basal prolactin secretion and may interfere with the establishment of lactation. Non-drug methods or, if medication is needed, nonsedating antihistamines are preferred alternatives. The British Society for Allergy and Clinical Immunology recommends cetirizine at the lowest dose as a preferred antihistamine in breast-feeding women. Loratadine is also usually considered compatible with breast-feeding. Consider the benefits of breast-feeding, the risk of potential infant drug exposure, and the risk of an untreated or inadequately treated condition. If a breast-feeding infant experiences an adverse effect related to a maternally ingested drug, healthcare providers are encouraged to report the adverse effect to the FDA.

    Closed-angle glaucoma, contact lenses

    Clemastine should be avoided, if possible, in patients with closed-angle glaucoma; an H1-antagonist with less anticholinergic effects should be substituted. An increase in intraocular pressure may occur from the anticholinergic actions of the drug, precipitating an acute attack of glaucoma. Elderly patients are more susceptible to the anticholinergic effects of clemastine, including possible precipitation of undiagnosed glaucoma. Other ocular effects resulting from the anticholinergic effects of clemastine include dry eyes or blurred vision. This may be of significance in the elderly and wearers of contact lenses.

    Bladder obstruction, GI obstruction, ileus, prostatic hypertrophy, urinary retention

    Clemastine has substantial anticholinergic effects and should be avoided in patients with bladder obstruction, GI obstruction or ileus, benign prostatic hypertrophy, or urinary retention. These precautions are most significant when using H1-antagonists from the ethanolamine and phenothiazine groups; therefore, an H1-antagonist with less anticholinergic effects should be substituted if treatment is required. The elderly are more susceptible to the anticholinergic effects of drugs since there is a decline in endogenous cholinergic activity that occurs with age.

    Hepatic disease

    Clemastine is extensively metabolized in the liver. The metabolism of clemastine may be reduced in the presence of hepatic impairment. Those with significant hepatic disease receiving clemastine should be monitored for liver function and side effects. Dosage reduction may be required in these patients. The use of an antihistamine with a shorter half-life may preferable in those with hepatic impairment.

    Cardiac disease

    The quinidine-like local anesthetic and anticholinergic effects of H1-antagonists are responsible for the adverse cardiac effects which have been observed including tachycardia, ECG changes, hypotension, and arrhythmias. Although these cardiovascular effects are uncommon, H1-antagonists should be used conservatively in patients with cardiac disease.

    Driving or operating machinery

    Clemastine can cause drowsiness. Patients receiving clemastine should be advised to avoid driving or operating machinery until the effects of the drug are known.

    Anticholinergic medications, geriatric

    Caution is advisable when using clemastine in geriatric adults because they may be more sensitive to the anticholinergic effects of clemastine than younger adults. In addition, the anticholinergic effects of clemastine may be significant and are additive with other anticholinergic medications, particularly in the elderly. According to the Beers Criteria, first generation sedating antihistamines, including clemastine, are considered potentially inappropriate medications (PIMs) for use in geriatric patients and should be avoided because they are highly anticholinergic, there is reduced clearance in advanced age, tolerance develops when used as hypnotics, and there is a greater risk of anticholinergic effects including confusion, dry mouth, constipation, and other anticholinergic actions and toxicity compared to younger adults. The Beers expert panel recommends avoiding drugs with strong anticholinergic properties, such as clemastine, in geriatric patients with the following disease states or symptoms due to the potential for exacerbation of the condition or increased risk of adverse effects: dementia/cognitive impairment (adverse CNS effects), delirium/high risk of delirium (possible new-onset or worsening delirium), or lower urinary tract symptoms/benign prostatic hyperplasia in men (possible urinary retention or hesitancy). The federal Omnibus Budget Reconciliation Act (OBRA) regulates medication use in residents of long-term care facilities. According to the OBRA guidelines, cough, cold, and allergy medications should be used only for a limited duration (less than 14 days) unless there is documented evidence of enduring symptoms that cannot otherwise be alleviated and for which a cause cannot be identified and corrected. Antihistamines (e.g., first generation agents) have strong anticholinergic properties and are not considered medications of choice in older individuals. If administered, antihistamines should be used in the smallest possible dose in individuals who are susceptible to anticholinergic side effects or who are receiving other medications with anticholinergic properties. Anticholinergics may cause excessive sedation, confusion, cognitive impairment, distress, dry mouth, constipation, and urinary retention. Many of these effects may lead to other adverse consequences, such as falls.

    ADVERSE REACTIONS

    Severe

    seizures / Delayed / Incidence not known
    anaphylactic shock / Rapid / Incidence not known
    hemolytic anemia / Delayed / Incidence not known
    agranulocytosis / Delayed / Incidence not known

    Moderate

    neuritis / Delayed / Incidence not known
    excitability / Early / Incidence not known
    euphoria / Early / Incidence not known
    impaired cognition / Early / Incidence not known
    confusion / Early / Incidence not known
    palpitations / Early / Incidence not known
    blurred vision / Early / Incidence not known
    urinary retention / Early / Incidence not known
    constipation / Delayed / Incidence not known
    hypotension / Rapid / Incidence not known
    sinus tachycardia / Rapid / Incidence not known
    thrombocytopenia / Delayed / Incidence not known

    Mild

    restlessness / Early / Incidence not known
    tinnitus / Delayed / Incidence not known
    drowsiness / Early / Incidence not known
    fatigue / Early / Incidence not known
    weakness / Early / Incidence not known
    headache / Early / Incidence not known
    dizziness / Early / Incidence not known
    insomnia / Early / Incidence not known
    tremor / Early / Incidence not known
    vertigo / Early / Incidence not known
    paresthesias / Delayed / Incidence not known
    irritability / Delayed / Incidence not known
    diplopia / Early / Incidence not known
    xerostomia / Early / Incidence not known
    increased urinary frequency / Early / Incidence not known
    xerophthalmia / Early / Incidence not known
    anorexia / Delayed / Incidence not known
    abdominal pain / Early / Incidence not known
    nausea / Early / Incidence not known
    diarrhea / Early / Incidence not known
    vomiting / Early / Incidence not known
    hyperhidrosis / Delayed / Incidence not known
    urticaria / Rapid / Incidence not known
    photosensitivity / Delayed / Incidence not known
    chills / Rapid / Incidence not known

    DRUG INTERACTIONS

    Acetaminophen; Butalbital: (Moderate) Additive CNS depression may occur if barbiturates are co-used with sedating antihistamines, such as clemastine. Monitor for additive CNS and respiratory effects, and warn about the potential effects to driving and other activities.
    Acetaminophen; Butalbital; Caffeine: (Moderate) Additive CNS depression may occur if barbiturates are co-used with sedating antihistamines, such as clemastine. Monitor for additive CNS and respiratory effects, and warn about the potential effects to driving and other activities.
    Acetaminophen; Butalbital; Caffeine; Codeine: (Moderate) Additive CNS depression may occur if barbiturates are co-used with sedating antihistamines, such as clemastine. Monitor for additive CNS and respiratory effects, and warn about the potential effects to driving and other activities. (Moderate) Concomitant use of codeine with sedating H1 blockers can potentiate respiratory depression and sedation. In addition, chlorpheniramine and diphenhydramine inhibit CYP2D6, an enzyme responsible for the metabolism of codeine. Monitor patients on these combinations closely.
    Acetaminophen; Caffeine; Dihydrocodeine: (Moderate) Concomitant use of dihydrocodeine containing products with sedating H1-blockers can potentiate respiratory depression and sedation. In addition, chlorpheniramine and diphenhydramine inhibit CYP2D6, an enzyme responsible for the metabolism of dihydrocodeine. Close monitoring for side effects in patients receiving dihydrocodeine containing products and chlorpheniramine or diphenhydramine is recommended.
    Acetaminophen; Codeine: (Moderate) Concomitant use of codeine with sedating H1 blockers can potentiate respiratory depression and sedation. In addition, chlorpheniramine and diphenhydramine inhibit CYP2D6, an enzyme responsible for the metabolism of codeine. Monitor patients on these combinations closely.
    Acetaminophen; Dichloralphenazone; Isometheptene: (Moderate) Additive CNS depression may occur if dichloralphenazone is used concomitantly with any of the sedating H1 blockers. Use caution with this combination. Dosage reduction of one or both agents may be necessary.
    Acetaminophen; Hydrocodone: (Moderate) Concomitant use of hydrocodone with other CNS depressants may lead to hypotension, profound sedation, coma, respiratory depression and death. Prior to concurrent use of hydrocodone in patients taking a CNS depressant, assess the level of tolerance to CNS depression that has developed, the duration of use, and the patient's overall response to treatment. Consider the patient's use of alcohol or illicit drugs. Hydrocodone should be used in reduced dosages if used concurrently with a CNS depressant; initiate hydrocodone at 20 to 30% of the usual dosage in patients that are concurrently receiving another CNS depressant. Also, consider a using a lower dose of the CNS depressant. Monitor patients for sedation and respiratory depression. In addition, the metabolism of hydrocodone to its active metabolite, hydromorphone, is dependent on CYP2D6. Theoretically, coadministration of hydrocodone and a CYP2D6 inhibitor, such as chlorpheniramine or diphenhydramine, may result in a reduction in the analgesic effect of hydrocodone.
    Acetaminophen; Oxycodone: (Moderate) Concomitant use of oxycodone with sedating H1-blockers may lead to additive respiratory and/or CNS depression. Hypotension, profound sedation, coma, respiratory depression, or death may occur. Prior to concurrent use of oxycodone in patients taking a CNS depressant, assess the level of tolerance to CNS depression that has developed, the duration of use, and the patient's overall response to treatment. Consider the patient's use of alcohol or illicit drugs. If a CNS depressant is used concurrently with oxycodone, a reduced dosage of oxycodone and/or the CNS depressant is recommended; use an initial dose of oxycodone at 1/3 to 1/2 the usual dosage. Monitor for sedation and respiratory depression.
    Acetaminophen; Pentazocine: (Moderate) Use pentazocine with caution in any patient receiving medication with CNS depressant and/or anticholinergic activity. Coadministration of pentazocine with sedating H1-blockers may result in additive respiratory and CNS depression and anticholinergic effects, such as urinary retention and constipation.
    Acetaminophen; Propoxyphene: (Moderate) Concomitant use of propoxyphene with other CNS depressants can potentiate respiratory depression and, or sedation. In addition, chlorpheniramine and diphenhydramine inhibit CYP2D6, an enzyme responsible for the metabolism of propoxyphene. Monitor these patients. Overdosage of propoxyphene in combination with other potent CNS depressants is a major cause of drug-related death; fatalities within the first hour of overdosage are not uncommon.
    Acetaminophen; Tramadol: (Moderate) An enhanced CNS depressant effect may occur when sedating h1-blockers are combined with other CNS depressants including tramadol.
    Alfentanil: (Moderate) Concomitant use of alfentanil with other CNS depressants, such as sedating H1 blockers can potentiate the effects of alfentanil on respiration, alertness, and blood pressure. A dose reduction of one or both drugs may be warranted.
    Alosetron: (Moderate) Alosetron, if combined with drugs that possess anticholinergic properties like sedating H1 blockers, may seriously worsen constipation, leading to events such as GI obstruction/impaction or paralytic ileus.
    Amantadine: (Moderate) Medications with significant anticholinergic activity may potentiate the anticholinergic effects of amantadine, and may increase the risk of antimuscarinic-related side effects. Additive drowsiness may also occur.
    Ambenonium Chloride: (Moderate) The therapeutic benefits of ambenonium may be diminished when coadministered with drugs known to exhibit anticholinergic properties including sedating H1-blockers. When concurrent use cannot be avoided, monitor the patient for reduced ambenonium efficacy.
    Amobarbital: (Moderate) Additive CNS depression may occur if barbiturates are co-used with sedating antihistamines, such as clemastine. Monitor for additive CNS and respiratory effects, and warn about the potential effects to driving and other activities.
    Amoxapine: (Moderate) Additive anticholinergic effects may be seen when amoxapine is used concomitantly with drugs are known to possess relatively significant antimuscarinic properties, including sedating h1-blockers. Clinicians should note that antimuscarinic effects might be seen not only on GI smooth muscle, but also on bladder function, the eye, and temperature Additive sedation may also occur.
    Amphetamine: (Moderate) Amphetamines may pharmacodynamically counteract the sedative properties of some antihistamines, such as the sedating H1-blockers (i.e., diphenhydramine). This effect may be clinically important if a patient is receiving an antihistamine agent for treatment of insomnia. Alternatively, if a patient is receiving an amphetamine for treatment of narcolepsy, the combination with a sedating antihistamine may reverse the action of the amphetamine.
    Amphetamine; Dextroamphetamine Salts: (Moderate) Amphetamines may pharmacodynamically counteract the sedative properties of some antihistamines, such as the sedating H1-blockers (i.e., diphenhydramine). This effect may be clinically important if a patient is receiving an antihistamine agent for treatment of insomnia. Alternatively, if a patient is receiving an amphetamine for treatment of narcolepsy, the combination with a sedating antihistamine may reverse the action of the amphetamine.
    Amphetamine; Dextroamphetamine: (Moderate) Amphetamines may pharmacodynamically counteract the sedative properties of some antihistamines, such as the sedating H1-blockers (i.e., diphenhydramine). This effect may be clinically important if a patient is receiving an antihistamine agent for treatment of insomnia. Alternatively, if a patient is receiving an amphetamine for treatment of narcolepsy, the combination with a sedating antihistamine may reverse the action of the amphetamine.
    Anticholinergics: (Moderate) The anticholinergic effects of sedating H1-blockers may be enhanced when combined with other antimuscarinics. Clinicians should note that anticholinergic effects might be seen not only on GI smooth muscle, but also on bladder function, the eye, and temperature regulation. Additive drowsiness may also occur when antimuscarinics are combined with sedating antihistamines.
    Apomorphine: (Moderate) Apomorphine causes significant somnolence. Concomitant administration of apomorphine and clemastine could result in additive depressant effects. Careful monitoring is recommended during combined use. A dose reduction of one or both drugs may be warranted.
    Aripiprazole: (Moderate) Due to the primary CNS effects of aripiprazole, caution should be used when aripiprazole is given in combination with other centrally-acting medications including sedating H1-blockers. Additive drowsiness or other CNS effects may occur.
    Asenapine: (Moderate) Using drugs that can cause CNS depression, such as sedating H1-blockers, concomitantly with asenapine may increase both the frequency and the intensity of adverse effects such as drowsiness, sedation, and dizziness.
    Aspirin, ASA; Butalbital; Caffeine: (Moderate) Additive CNS depression may occur if barbiturates are co-used with sedating antihistamines, such as clemastine. Monitor for additive CNS and respiratory effects, and warn about the potential effects to driving and other activities.
    Aspirin, ASA; Butalbital; Caffeine; Codeine: (Moderate) Additive CNS depression may occur if barbiturates are co-used with sedating antihistamines, such as clemastine. Monitor for additive CNS and respiratory effects, and warn about the potential effects to driving and other activities. (Moderate) Concomitant use of codeine with sedating H1 blockers can potentiate respiratory depression and sedation. In addition, chlorpheniramine and diphenhydramine inhibit CYP2D6, an enzyme responsible for the metabolism of codeine. Monitor patients on these combinations closely.
    Aspirin, ASA; Caffeine; Dihydrocodeine: (Moderate) Concomitant use of dihydrocodeine containing products with sedating H1-blockers can potentiate respiratory depression and sedation. In addition, chlorpheniramine and diphenhydramine inhibit CYP2D6, an enzyme responsible for the metabolism of dihydrocodeine. Close monitoring for side effects in patients receiving dihydrocodeine containing products and chlorpheniramine or diphenhydramine is recommended.
    Aspirin, ASA; Carisoprodol: (Moderate) Carisoprodol is metabolized to meprobamate, a significant CNS depressant. Carisoprodol can cause additive CNS depression if used concomitantly with other CNS depressants. Additive effects of sedation and dizziness, which can impair the ability to undertake tasks requiring mental alertness, may occur if carisoprodol is taken with sedating H1-blockers. Utilize appropriate caution if carisoprodol is coadministered with another CNS depressant.
    Aspirin, ASA; Carisoprodol; Codeine: (Moderate) Carisoprodol is metabolized to meprobamate, a significant CNS depressant. Carisoprodol can cause additive CNS depression if used concomitantly with other CNS depressants. Additive effects of sedation and dizziness, which can impair the ability to undertake tasks requiring mental alertness, may occur if carisoprodol is taken with sedating H1-blockers. Utilize appropriate caution if carisoprodol is coadministered with another CNS depressant. (Moderate) Concomitant use of codeine with sedating H1 blockers can potentiate respiratory depression and sedation. In addition, chlorpheniramine and diphenhydramine inhibit CYP2D6, an enzyme responsible for the metabolism of codeine. Monitor patients on these combinations closely.
    Aspirin, ASA; Oxycodone: (Moderate) Concomitant use of oxycodone with sedating H1-blockers may lead to additive respiratory and/or CNS depression. Hypotension, profound sedation, coma, respiratory depression, or death may occur. Prior to concurrent use of oxycodone in patients taking a CNS depressant, assess the level of tolerance to CNS depression that has developed, the duration of use, and the patient's overall response to treatment. Consider the patient's use of alcohol or illicit drugs. If a CNS depressant is used concurrently with oxycodone, a reduced dosage of oxycodone and/or the CNS depressant is recommended; use an initial dose of oxycodone at 1/3 to 1/2 the usual dosage. Monitor for sedation and respiratory depression.
    Atracurium: (Moderate) An enhanced CNS depressant effect may occur when sedating H1-blockers are combined with other CNS depressants including neuromuscular blockers.
    Atropine; Difenoxin: (Moderate) An enhanced CNS depressant effect may occur when diphenoxylate/difenoxin is combined with other CNS depressants. Diphenoxylate/difenoxin decreases GI motility. Other drugs that also decrease GI motility, such as sedating H1 blockers, may produce additive effects with diphenoxylate/difenoxin if used concomitantly.
    Atropine; Diphenoxylate: (Moderate) An enhanced CNS depressant effect may occur when diphenoxylate/difenoxin is combined with other CNS depressants. Diphenoxylate/difenoxin decreases GI motility. Other drugs that also decrease GI motility, such as sedating H1 blockers, may produce additive effects with diphenoxylate/difenoxin if used concomitantly.
    Atropine; Hyoscyamine; Phenobarbital; Scopolamine: (Moderate) Additive CNS depression may occur if barbiturates are co-used with sedating antihistamines, such as clemastine. Monitor for additive CNS and respiratory effects, and warn about the potential effects to driving and other activities.
    Azelastine: (Major) An enhanced CNS depressant effect may occur when azelastine is combined with other CNS depressants including sedating H1-blockers; avoid concurrent use.
    Azelastine; Fluticasone: (Major) An enhanced CNS depressant effect may occur when azelastine is combined with other CNS depressants including sedating H1-blockers; avoid concurrent use.
    Baclofen: (Moderate) An enhanced CNS depressant effect may occur when sedating H1-blockers are combined with other CNS depressants including skeletal muscle relaxants, such as baclofen.
    Barbiturates: (Moderate) Additive CNS depression may occur if barbiturates are co-used with sedating antihistamines, such as clemastine. Monitor for additive CNS and respiratory effects, and warn about the potential effects to driving and other activities.
    Belladonna Alkaloids; Ergotamine; Phenobarbital: (Moderate) Additive CNS depression may occur if barbiturates are co-used with sedating antihistamines, such as clemastine. Monitor for additive CNS and respiratory effects, and warn about the potential effects to driving and other activities.
    Belladonna; Opium: (Moderate) Enhanced CNS depressant effects may occur when opium is combined with other CNS depressants, such as sedating H1 blockers.
    Benzodiazepines: (Moderate) Coadministration can potentiate the CNS effects (e.g., increased sedation or respiratory depression) of either agent. Use caution with this combination.
    Benzphetamine: (Moderate) Amphetamines may pharmacodynamically counteract the sedative properties of some antihistamines, such as the sedating H1-blockers. This effect may be clinically important if a patient is receiving an antihistamine agent for treatment of insomnia. Alternatively, if a patient is receiving an amphetamine for treatment of narcolepsy, the combination with a sedating antihistamine may reverse the action of the amphetamine.
    Bethanechol: (Moderate) Drugs that possess antimuscarinic properties, such as clemastine, are pharmacologic opposites of bethanechol. These agents should not be used with bethanechol except when the specific intent is to counteract excessive actions of one or the other.
    Brompheniramine; Carbetapentane; Phenylephrine: (Moderate) Drowsiness has been reported during administration of carbetapentane. An enhanced CNS depressant effect may occur when carbetapentane is combined with other CNS depressants including sedating h1-blockers.
    Brompheniramine; Guaifenesin; Hydrocodone: (Moderate) Concomitant use of hydrocodone with other CNS depressants may lead to hypotension, profound sedation, coma, respiratory depression and death. Prior to concurrent use of hydrocodone in patients taking a CNS depressant, assess the level of tolerance to CNS depression that has developed, the duration of use, and the patient's overall response to treatment. Consider the patient's use of alcohol or illicit drugs. Hydrocodone should be used in reduced dosages if used concurrently with a CNS depressant; initiate hydrocodone at 20 to 30% of the usual dosage in patients that are concurrently receiving another CNS depressant. Also, consider a using a lower dose of the CNS depressant. Monitor patients for sedation and respiratory depression. In addition, the metabolism of hydrocodone to its active metabolite, hydromorphone, is dependent on CYP2D6. Theoretically, coadministration of hydrocodone and a CYP2D6 inhibitor, such as chlorpheniramine or diphenhydramine, may result in a reduction in the analgesic effect of hydrocodone.
    Brompheniramine; Hydrocodone; Pseudoephedrine: (Moderate) Concomitant use of hydrocodone with other CNS depressants may lead to hypotension, profound sedation, coma, respiratory depression and death. Prior to concurrent use of hydrocodone in patients taking a CNS depressant, assess the level of tolerance to CNS depression that has developed, the duration of use, and the patient's overall response to treatment. Consider the patient's use of alcohol or illicit drugs. Hydrocodone should be used in reduced dosages if used concurrently with a CNS depressant; initiate hydrocodone at 20 to 30% of the usual dosage in patients that are concurrently receiving another CNS depressant. Also, consider a using a lower dose of the CNS depressant. Monitor patients for sedation and respiratory depression. In addition, the metabolism of hydrocodone to its active metabolite, hydromorphone, is dependent on CYP2D6. Theoretically, coadministration of hydrocodone and a CYP2D6 inhibitor, such as chlorpheniramine or diphenhydramine, may result in a reduction in the analgesic effect of hydrocodone.
    Buprenorphine: (Moderate) If concurrent use of sedating H1-blockers and buprenorphine is necessary, consider a dose reduction of one or both drugs because of the potential for additive pharmacological effects. Hypotension, profound sedation, coma, respiratory depression, or death may occur during co-administration of buprenorphine and other CNS depressants. Prior to concurrent use of buprenorphine in patients taking a CNS depressant, assess the level of tolerance to CNS depression that has developed, the duration of use, and the patient's overall response to treatment. Evaluate the patient's use of alcohol or illicit drugs. It is recommended that the injectable buprenorphine dose be halved for patients who receive other drugs with CNS depressant effects; for the buprenorphine transdermal patch, start with the 5 mcg/hour patch. Monitor patients for sedation or respiratory depression.
    Buprenorphine; Naloxone: (Moderate) If concurrent use of sedating H1-blockers and buprenorphine is necessary, consider a dose reduction of one or both drugs because of the potential for additive pharmacological effects. Hypotension, profound sedation, coma, respiratory depression, or death may occur during co-administration of buprenorphine and other CNS depressants. Prior to concurrent use of buprenorphine in patients taking a CNS depressant, assess the level of tolerance to CNS depression that has developed, the duration of use, and the patient's overall response to treatment. Evaluate the patient's use of alcohol or illicit drugs. It is recommended that the injectable buprenorphine dose be halved for patients who receive other drugs with CNS depressant effects; for the buprenorphine transdermal patch, start with the 5 mcg/hour patch. Monitor patients for sedation or respiratory depression.
    Buspirone: (Moderate) The combination of buspirone and other CNS depressants, such as sedating h1-blockers, can increase the risk for sedation.
    Butabarbital: (Moderate) Additive CNS depression may occur if barbiturates are co-used with sedating antihistamines, such as clemastine. Monitor for additive CNS and respiratory effects, and warn about the potential effects to driving and other activities.
    Butorphanol: (Moderate) Concomitant use of butorphanol with sedating H1-blockers can potentiate the effects of butorphanol on CNS and/or respiratory depression. Use together with caution. If a CNS depressant needs to be used with butorphanol, use the smallest effective dose and the longest dosing frequency of butorphanol.
    Capsaicin; Metaxalone: (Moderate) Concomitant administration of metaxalone with other CNS depressants can potentiate the sedative effects of either agent.
    Carbetapentane; Chlorpheniramine: (Moderate) Drowsiness has been reported during administration of carbetapentane. An enhanced CNS depressant effect may occur when carbetapentane is combined with other CNS depressants including sedating h1-blockers.
    Carbetapentane; Chlorpheniramine; Phenylephrine: (Moderate) Drowsiness has been reported during administration of carbetapentane. An enhanced CNS depressant effect may occur when carbetapentane is combined with other CNS depressants including sedating h1-blockers.
    Carbetapentane; Diphenhydramine; Phenylephrine: (Moderate) Drowsiness has been reported during administration of carbetapentane. An enhanced CNS depressant effect may occur when carbetapentane is combined with other CNS depressants including sedating h1-blockers.
    Carbetapentane; Guaifenesin: (Moderate) Drowsiness has been reported during administration of carbetapentane. An enhanced CNS depressant effect may occur when carbetapentane is combined with other CNS depressants including sedating h1-blockers.
    Carbetapentane; Guaifenesin; Phenylephrine: (Moderate) Drowsiness has been reported during administration of carbetapentane. An enhanced CNS depressant effect may occur when carbetapentane is combined with other CNS depressants including sedating h1-blockers.
    Carbetapentane; Phenylephrine: (Moderate) Drowsiness has been reported during administration of carbetapentane. An enhanced CNS depressant effect may occur when carbetapentane is combined with other CNS depressants including sedating h1-blockers.
    Carbetapentane; Phenylephrine; Pyrilamine: (Moderate) Drowsiness has been reported during administration of carbetapentane. An enhanced CNS depressant effect may occur when carbetapentane is combined with other CNS depressants including sedating h1-blockers.
    Carbetapentane; Pseudoephedrine: (Moderate) Drowsiness has been reported during administration of carbetapentane. An enhanced CNS depressant effect may occur when carbetapentane is combined with other CNS depressants including sedating h1-blockers.
    Carbetapentane; Pyrilamine: (Moderate) Drowsiness has been reported during administration of carbetapentane. An enhanced CNS depressant effect may occur when carbetapentane is combined with other CNS depressants including sedating h1-blockers.
    Carbidopa; Levodopa; Entacapone: (Moderate) COMT inhibitors, such as entacapone or tolcapone, should be given cautiously with other agents that cause CNS depression, including sedating H1-blockers, due to the possibility of additive sedation.
    Carbinoxamine; Hydrocodone; Phenylephrine: (Moderate) Concomitant use of hydrocodone with other CNS depressants may lead to hypotension, profound sedation, coma, respiratory depression and death. Prior to concurrent use of hydrocodone in patients taking a CNS depressant, assess the level of tolerance to CNS depression that has developed, the duration of use, and the patient's overall response to treatment. Consider the patient's use of alcohol or illicit drugs. Hydrocodone should be used in reduced dosages if used concurrently with a CNS depressant; initiate hydrocodone at 20 to 30% of the usual dosage in patients that are concurrently receiving another CNS depressant. Also, consider a using a lower dose of the CNS depressant. Monitor patients for sedation and respiratory depression. In addition, the metabolism of hydrocodone to its active metabolite, hydromorphone, is dependent on CYP2D6. Theoretically, coadministration of hydrocodone and a CYP2D6 inhibitor, such as chlorpheniramine or diphenhydramine, may result in a reduction in the analgesic effect of hydrocodone.
    Carbinoxamine; Hydrocodone; Pseudoephedrine: (Moderate) Concomitant use of hydrocodone with other CNS depressants may lead to hypotension, profound sedation, coma, respiratory depression and death. Prior to concurrent use of hydrocodone in patients taking a CNS depressant, assess the level of tolerance to CNS depression that has developed, the duration of use, and the patient's overall response to treatment. Consider the patient's use of alcohol or illicit drugs. Hydrocodone should be used in reduced dosages if used concurrently with a CNS depressant; initiate hydrocodone at 20 to 30% of the usual dosage in patients that are concurrently receiving another CNS depressant. Also, consider a using a lower dose of the CNS depressant. Monitor patients for sedation and respiratory depression. In addition, the metabolism of hydrocodone to its active metabolite, hydromorphone, is dependent on CYP2D6. Theoretically, coadministration of hydrocodone and a CYP2D6 inhibitor, such as chlorpheniramine or diphenhydramine, may result in a reduction in the analgesic effect of hydrocodone.
    Carisoprodol: (Moderate) Carisoprodol is metabolized to meprobamate, a significant CNS depressant. Carisoprodol can cause additive CNS depression if used concomitantly with other CNS depressants. Additive effects of sedation and dizziness, which can impair the ability to undertake tasks requiring mental alertness, may occur if carisoprodol is taken with sedating H1-blockers. Utilize appropriate caution if carisoprodol is coadministered with another CNS depressant.
    Cetirizine: (Major) Dry mouth, drowsiness and other antihistamine-related side effects may occur in patients receiving cetirizine. Due to the duplicative and additive nature of the pharmacology of cetirizine, concurrent use of sedating antihistamines (H1-blockers) is not recommended.
    Cetirizine; Pseudoephedrine: (Major) Dry mouth, drowsiness and other antihistamine-related side effects may occur in patients receiving cetirizine. Due to the duplicative and additive nature of the pharmacology of cetirizine, concurrent use of sedating antihistamines (H1-blockers) is not recommended.
    Chlorpheniramine; Codeine: (Moderate) Concomitant use of codeine with sedating H1 blockers can potentiate respiratory depression and sedation. In addition, chlorpheniramine and diphenhydramine inhibit CYP2D6, an enzyme responsible for the metabolism of codeine. Monitor patients on these combinations closely.
    Chlorpheniramine; Dihydrocodeine; Phenylephrine: (Moderate) Concomitant use of dihydrocodeine containing products with sedating H1-blockers can potentiate respiratory depression and sedation. In addition, chlorpheniramine and diphenhydramine inhibit CYP2D6, an enzyme responsible for the metabolism of dihydrocodeine. Close monitoring for side effects in patients receiving dihydrocodeine containing products and chlorpheniramine or diphenhydramine is recommended.
    Chlorpheniramine; Dihydrocodeine; Pseudoephedrine: (Moderate) Concomitant use of dihydrocodeine containing products with sedating H1-blockers can potentiate respiratory depression and sedation. In addition, chlorpheniramine and diphenhydramine inhibit CYP2D6, an enzyme responsible for the metabolism of dihydrocodeine. Close monitoring for side effects in patients receiving dihydrocodeine containing products and chlorpheniramine or diphenhydramine is recommended.
    Chlorpheniramine; Guaifenesin; Hydrocodone; Pseudoephedrine: (Moderate) Concomitant use of hydrocodone with other CNS depressants may lead to hypotension, profound sedation, coma, respiratory depression and death. Prior to concurrent use of hydrocodone in patients taking a CNS depressant, assess the level of tolerance to CNS depression that has developed, the duration of use, and the patient's overall response to treatment. Consider the patient's use of alcohol or illicit drugs. Hydrocodone should be used in reduced dosages if used concurrently with a CNS depressant; initiate hydrocodone at 20 to 30% of the usual dosage in patients that are concurrently receiving another CNS depressant. Also, consider a using a lower dose of the CNS depressant. Monitor patients for sedation and respiratory depression. In addition, the metabolism of hydrocodone to its active metabolite, hydromorphone, is dependent on CYP2D6. Theoretically, coadministration of hydrocodone and a CYP2D6 inhibitor, such as chlorpheniramine or diphenhydramine, may result in a reduction in the analgesic effect of hydrocodone.
    Chlorpheniramine; Hydrocodone: (Moderate) Concomitant use of hydrocodone with other CNS depressants may lead to hypotension, profound sedation, coma, respiratory depression and death. Prior to concurrent use of hydrocodone in patients taking a CNS depressant, assess the level of tolerance to CNS depression that has developed, the duration of use, and the patient's overall response to treatment. Consider the patient's use of alcohol or illicit drugs. Hydrocodone should be used in reduced dosages if used concurrently with a CNS depressant; initiate hydrocodone at 20 to 30% of the usual dosage in patients that are concurrently receiving another CNS depressant. Also, consider a using a lower dose of the CNS depressant. Monitor patients for sedation and respiratory depression. In addition, the metabolism of hydrocodone to its active metabolite, hydromorphone, is dependent on CYP2D6. Theoretically, coadministration of hydrocodone and a CYP2D6 inhibitor, such as chlorpheniramine or diphenhydramine, may result in a reduction in the analgesic effect of hydrocodone.
    Chlorpheniramine; Hydrocodone; Phenylephrine: (Moderate) Concomitant use of hydrocodone with other CNS depressants may lead to hypotension, profound sedation, coma, respiratory depression and death. Prior to concurrent use of hydrocodone in patients taking a CNS depressant, assess the level of tolerance to CNS depression that has developed, the duration of use, and the patient's overall response to treatment. Consider the patient's use of alcohol or illicit drugs. Hydrocodone should be used in reduced dosages if used concurrently with a CNS depressant; initiate hydrocodone at 20 to 30% of the usual dosage in patients that are concurrently receiving another CNS depressant. Also, consider a using a lower dose of the CNS depressant. Monitor patients for sedation and respiratory depression. In addition, the metabolism of hydrocodone to its active metabolite, hydromorphone, is dependent on CYP2D6. Theoretically, coadministration of hydrocodone and a CYP2D6 inhibitor, such as chlorpheniramine or diphenhydramine, may result in a reduction in the analgesic effect of hydrocodone.
    Chlorpheniramine; Hydrocodone; Pseudoephedrine: (Moderate) Concomitant use of hydrocodone with other CNS depressants may lead to hypotension, profound sedation, coma, respiratory depression and death. Prior to concurrent use of hydrocodone in patients taking a CNS depressant, assess the level of tolerance to CNS depression that has developed, the duration of use, and the patient's overall response to treatment. Consider the patient's use of alcohol or illicit drugs. Hydrocodone should be used in reduced dosages if used concurrently with a CNS depressant; initiate hydrocodone at 20 to 30% of the usual dosage in patients that are concurrently receiving another CNS depressant. Also, consider a using a lower dose of the CNS depressant. Monitor patients for sedation and respiratory depression. In addition, the metabolism of hydrocodone to its active metabolite, hydromorphone, is dependent on CYP2D6. Theoretically, coadministration of hydrocodone and a CYP2D6 inhibitor, such as chlorpheniramine or diphenhydramine, may result in a reduction in the analgesic effect of hydrocodone.
    Chlorpromazine: (Moderate) Additive effects may be seen when phenothiazines are used concomitantly with other drugs with antimuscarinic activity, such as clemastine, a sedating H1-blocker. Phenothiazines, such as thioridazine and chlorpromazine, may exhibit pronounced anticholinergic and sedative effects. Clinicians should note that antimuscarinic effects may be seen not only on GI smooth muscle, but also on bladder function, the eye, and temperature regulation. Additive drowsiness or other CNS effects may also occur.
    Cisatracurium: (Moderate) An enhanced CNS depressant effect may occur when sedating H1-blockers are combined with other CNS depressants including neuromuscular blockers.
    Clobazam: (Moderate) Clobazam, a benzodiazepine, may cause drowsiness or other CNS effects. Additive drowsiness may occur when clobazam is combined with CNS depressants such as sedating H1-blockers. In addition, caution is recommended when administering clobazam with medications extensively metabolized by CYP2D6 such as diphenhydramine because clobazam has been shown to inhibit CYP2D6 in vivo and may increase concentrations of drugs metabolized by this enzyme.
    Clozapine: (Moderate) Clozapine exhibits anticholinergic effects that may be clinically significant. Clinicians should keep this in mind when using the antimuscarinics or other medications with anticholinergic activity in combination with clozapine. Some medications that exhibit additive anticholinergic effects include sedating H1-blockers. Clozapine may also cause additive sedation with many of these drugs. Clinicians should note that antimuscarinic effects might be seen not only on GI smooth muscle, but also on bladder function, the eye, and temperature regulation.
    Codeine: (Moderate) Concomitant use of codeine with sedating H1 blockers can potentiate respiratory depression and sedation. In addition, chlorpheniramine and diphenhydramine inhibit CYP2D6, an enzyme responsible for the metabolism of codeine. Monitor patients on these combinations closely.
    Codeine; Guaifenesin: (Moderate) Concomitant use of codeine with sedating H1 blockers can potentiate respiratory depression and sedation. In addition, chlorpheniramine and diphenhydramine inhibit CYP2D6, an enzyme responsible for the metabolism of codeine. Monitor patients on these combinations closely.
    Codeine; Phenylephrine; Promethazine: (Moderate) Additive anticholinergic effects may be seen when promethazine is used concomitantly with other drugs with antimuscarinic activity like sedating H1-blockers. Clinicians should note that antimuscarinic effects may be seen not only on GI smooth muscle, but also on bladder function, the eye, and temperature regulation. Because promethazine causes pronounced sedation, an enhanced CNS depressant effect or additive drowsiness may occur when it is combined with other CNS depressants including sedating H1-blockers. (Moderate) Concomitant use of codeine with sedating H1 blockers can potentiate respiratory depression and sedation. In addition, chlorpheniramine and diphenhydramine inhibit CYP2D6, an enzyme responsible for the metabolism of codeine. Monitor patients on these combinations closely.
    Codeine; Promethazine: (Moderate) Additive anticholinergic effects may be seen when promethazine is used concomitantly with other drugs with antimuscarinic activity like sedating H1-blockers. Clinicians should note that antimuscarinic effects may be seen not only on GI smooth muscle, but also on bladder function, the eye, and temperature regulation. Because promethazine causes pronounced sedation, an enhanced CNS depressant effect or additive drowsiness may occur when it is combined with other CNS depressants including sedating H1-blockers. (Moderate) Concomitant use of codeine with sedating H1 blockers can potentiate respiratory depression and sedation. In addition, chlorpheniramine and diphenhydramine inhibit CYP2D6, an enzyme responsible for the metabolism of codeine. Monitor patients on these combinations closely.
    COMT inhibitors: (Moderate) COMT inhibitors, such as entacapone or tolcapone, should be given cautiously with other agents that cause CNS depression, including sedating H1-blockers, due to the possibility of additive sedation.
    Cyclobenzaprine: (Moderate) Cyclobenzaprine possesses antimuscarinic properties, which can cause dry mouth, urinary difficulties and slowing of gastrointestinal motility. If used with other drugs with antimuscarinic properties, such as sedating H1-blockers, anticholinergic side effects can be additive. Particular attention should be paid to GI problems because of the possible development of paralytic ileus. Additive CNS depression causing sedation and/or dizziness is also possible. Dosage adjustments of either or both drugs may be necessary.
    Dantrolene: (Moderate) Because sedating H1-blockers cause sedation, an enhanced CNS depressant effect (e.g., drowsiness) may occur when dantrolene is combined with other CNS depressants.
    Desloratadine: (Minor) Although desloratadine is considered a 'non-sedating' antihistamine, dose-related sedation has been noted. For this reason, it would be prudent to monitor for drowsiness during concurrent use of desloratadine with CNS depressants such as other H1-blockers.
    Desloratadine; Pseudoephedrine: (Minor) Although desloratadine is considered a 'non-sedating' antihistamine, dose-related sedation has been noted. For this reason, it would be prudent to monitor for drowsiness during concurrent use of desloratadine with CNS depressants such as other H1-blockers.
    Deutetrabenazine: (Moderate) Advise patients that concurrent use of deutetrabenazine and drugs that can cause CNS depression, such as clemastine, may have additive effects and worsen drowsiness or sedation.
    Dexmedetomidine: (Moderate) Co-administration of dexmedetomidine with sedating antihistamines is likely to lead to an enhancement of CNS depression.
    Dextroamphetamine: (Moderate) Amphetamines may pharmacodynamically counteract the sedative properties of some antihistamines, such as the sedating H1-blockers (i.e., diphenhydramine). This effect may be clinically important if a patient is receiving an antihistamine agent for treatment of insomnia. Alternatively, if a patient is receiving an amphetamine for treatment of narcolepsy, the combination with a sedating antihistamine may reverse the action of the amphetamine.
    Dextromethorphan; Promethazine: (Moderate) Additive anticholinergic effects may be seen when promethazine is used concomitantly with other drugs with antimuscarinic activity like sedating H1-blockers. Clinicians should note that antimuscarinic effects may be seen not only on GI smooth muscle, but also on bladder function, the eye, and temperature regulation. Because promethazine causes pronounced sedation, an enhanced CNS depressant effect or additive drowsiness may occur when it is combined with other CNS depressants including sedating H1-blockers.
    Dihydrocodeine; Guaifenesin; Pseudoephedrine: (Moderate) Concomitant use of dihydrocodeine containing products with sedating H1-blockers can potentiate respiratory depression and sedation. In addition, chlorpheniramine and diphenhydramine inhibit CYP2D6, an enzyme responsible for the metabolism of dihydrocodeine. Close monitoring for side effects in patients receiving dihydrocodeine containing products and chlorpheniramine or diphenhydramine is recommended.
    Diphenhydramine; Hydrocodone; Phenylephrine: (Moderate) Concomitant use of hydrocodone with other CNS depressants may lead to hypotension, profound sedation, coma, respiratory depression and death. Prior to concurrent use of hydrocodone in patients taking a CNS depressant, assess the level of tolerance to CNS depression that has developed, the duration of use, and the patient's overall response to treatment. Consider the patient's use of alcohol or illicit drugs. Hydrocodone should be used in reduced dosages if used concurrently with a CNS depressant; initiate hydrocodone at 20 to 30% of the usual dosage in patients that are concurrently receiving another CNS depressant. Also, consider a using a lower dose of the CNS depressant. Monitor patients for sedation and respiratory depression. In addition, the metabolism of hydrocodone to its active metabolite, hydromorphone, is dependent on CYP2D6. Theoretically, coadministration of hydrocodone and a CYP2D6 inhibitor, such as chlorpheniramine or diphenhydramine, may result in a reduction in the analgesic effect of hydrocodone.
    Disopyramide: (Moderate) The anticholinergic effects of sedating H1-blockers may be enhanced when combined with other drugs with moderate to significant anticholinergic effects including disopyramide. Clinicians should note that antimuscarinic effects might be seen not only on GI smooth muscle, but also on bladder function, the eye, and temperature regulation. Additive drowsiness or other CNS effects may also occur.
    Donepezil: (Moderate) Concurrent use of sedating H1-blockers and donepezil should be avoided if possible. Donepezil inhibits acetylcholinesterase, the enzyme responsible for the degradation of acetylcholine, and improves the availability of acetylcholine. Sedating H1-blockers may exhibit significant anticholinergic activity, thereby interfering with the therapeutic effect of donepezil.
    Donepezil; Memantine: (Moderate) Concurrent use of sedating H1-blockers and donepezil should be avoided if possible. Donepezil inhibits acetylcholinesterase, the enzyme responsible for the degradation of acetylcholine, and improves the availability of acetylcholine. Sedating H1-blockers may exhibit significant anticholinergic activity, thereby interfering with the therapeutic effect of donepezil.
    Doxacurium: (Moderate) An enhanced CNS depressant effect may occur when sedating H1-blockers are combined with other CNS depressants including neuromuscular blockers.
    Dronabinol, THC: (Moderate) Use caution if coadministration of dronabinol with antihistamines is necessary. Concurrent use of dronabinol, THC with antihistamines may result in additive drowsiness, hypertension, tachycardia, and possibly cardiotoxicity.
    Droperidol: (Moderate) Sedating H1-blockers have additive or potentiating sedative and other CNS effects with droperidol. Following administration of droperidol, lower doses of the other CNS depressant may need to be used.
    Entacapone: (Moderate) COMT inhibitors, such as entacapone or tolcapone, should be given cautiously with other agents that cause CNS depression, including sedating H1-blockers, due to the possibility of additive sedation.
    Eszopiclone: (Moderate) A reduction in the dose of eszopiclone and concomitantly administered CNS depressants, such as sedating H1-blockers, should be considered to minimize additive sedative effects. In addition, the risk of next-day psychomotor impairment is increased during co-administration of eszopiclone and other CNS depressants, which may decrease the ability to perform tasks requiring full mental alertness such as driving.
    Ethanol: (Major) Alcohol is associated with CNS depression. The combined use of alcohol and CNS depressants can lead to additive CNS depression, which could be dangerous in tasks requiring mental alertness and fatal in overdose. Alcohol taken with other CNS depressants can lead to additive respiratory depression, hypotension, profound sedation, or coma. Consider the patient's use of alcohol or illicit drugs when prescribing CNS depressant medications. In many cases, the patient should receive a lower dose of the CNS depressant initially if the patient is not likely to be compliant with avoiding alcohol.
    Etomidate: (Minor) Because sedating H1-blockers cause sedation, an enhanced CNS depressant effect may occur when they are combined with general anesthetics.
    Ezogabine: (Moderate) Caution is advisable during concurrent use of ezogabine and medications that may affect voiding such as clemastine, a sedating antihistamine (H1-blocker). Ezogabine has caused urinary retention requiring catheterization in some cases. The anticholinergic effects of clemastine on the urinary tract may be additive. Additive sedation or other CNS effects may also occur.
    Fentanyl: (Major) Avoid coadministration of fentanyl with other CNS depressants when possible, as this significantly increases the risk for profound sedation, respiratory depression, hypotension, coma, and death. Reserve concomitant use of these drugs for patients in whom alternative treatment options are inadequate. If concurrent use is necessary, use the lowest effective doses and minimum treatment durations possible and monitor patients closely for signs and symptoms of respiratory depression and sedation.
    Flibanserin: (Moderate) The concomitant use of flibanserin with CNS depressants, such as sedating H1-blockers, may increase the risk of CNS depression (e.g., dizziness, somnolence) compared to the use of flibanserin alone. Patients should avoid activities requiring full alertness (e.g., operating machinery or driving) until at least 6 hours after each dose and until they know how flibanserin affects them.
    Fluoxetine; Olanzapine: (Moderate) Olanzapine exhibits anticholinergic effects that may be clinically significant. Clinicians should keep this in mind when using antimuscarinics and other medications with anticholinergic activity in combination with olanzapine. Some medications exhibit additive anticholinergic effects include sedating H1-blockers. Olanzapine may also cause additive sedation with many of these drugs.
    Fluphenazine: (Moderate) Additive effects may be seen when phenothiazines are used concomitantly with other drugs with antimuscarinic activity, such as clemastine, a sedating H1-blocker. Phenothiazines, such as thioridazine and chlorpromazine, may exhibit pronounced anticholinergic and sedative effects. Clinicians should note that antimuscarinic effects may be seen not only on GI smooth muscle, but also on bladder function, the eye, and temperature regulation. Additive drowsiness or other CNS effects may also occur.
    Fospropofol: (Minor) Because sedating H1-blockers cause sedation, an enhanced CNS depressant effect may occur when they are combined with general anesthetics like fospropofol.
    Gabapentin: (Moderate) Coadministration of gabapentin with anxiolytics, sedatives, and hypnotics may increase CNS depressive effects such as drowsiness and dizziness. Use caution when administering gabapentin with CNS depressants. Patients should limit activity until they are aware of how coadministration affects them.
    Galantamine: (Moderate) Concurrent use of sedating H1-blockers and galantamine should be avoided if possible. Galantamine inhibits acetylcholinesterase, the enzyme responsible for the degradation of acetylcholine, and improves the availability of acetylcholine. Sedating H1-blockers may exhibit significant anticholinergic activity, thereby interfering with the therapeutic effect of galantamine.
    Guaifenesin; Hydrocodone: (Moderate) Concomitant use of hydrocodone with other CNS depressants may lead to hypotension, profound sedation, coma, respiratory depression and death. Prior to concurrent use of hydrocodone in patients taking a CNS depressant, assess the level of tolerance to CNS depression that has developed, the duration of use, and the patient's overall response to treatment. Consider the patient's use of alcohol or illicit drugs. Hydrocodone should be used in reduced dosages if used concurrently with a CNS depressant; initiate hydrocodone at 20 to 30% of the usual dosage in patients that are concurrently receiving another CNS depressant. Also, consider a using a lower dose of the CNS depressant. Monitor patients for sedation and respiratory depression. In addition, the metabolism of hydrocodone to its active metabolite, hydromorphone, is dependent on CYP2D6. Theoretically, coadministration of hydrocodone and a CYP2D6 inhibitor, such as chlorpheniramine or diphenhydramine, may result in a reduction in the analgesic effect of hydrocodone.
    Guaifenesin; Hydrocodone; Pseudoephedrine: (Moderate) Concomitant use of hydrocodone with other CNS depressants may lead to hypotension, profound sedation, coma, respiratory depression and death. Prior to concurrent use of hydrocodone in patients taking a CNS depressant, assess the level of tolerance to CNS depression that has developed, the duration of use, and the patient's overall response to treatment. Consider the patient's use of alcohol or illicit drugs. Hydrocodone should be used in reduced dosages if used concurrently with a CNS depressant; initiate hydrocodone at 20 to 30% of the usual dosage in patients that are concurrently receiving another CNS depressant. Also, consider a using a lower dose of the CNS depressant. Monitor patients for sedation and respiratory depression. In addition, the metabolism of hydrocodone to its active metabolite, hydromorphone, is dependent on CYP2D6. Theoretically, coadministration of hydrocodone and a CYP2D6 inhibitor, such as chlorpheniramine or diphenhydramine, may result in a reduction in the analgesic effect of hydrocodone.
    Halogenated Anesthetics: (Minor) Because sedating H1-blockers cause sedation, an enhanced CNS depressant effect may occur when they are combined with general anesthetics.
    Haloperidol: (Moderate) Haloperidol can potentiate the actions of other CNS depressants such as the sedating H1-blockers. Additive anticholinergic effects may occur. Clinicians should note that antimuscarinic effects may be seen not only on GI smooth muscle, but also on bladder function, the eye, and temperature regulation. Additive drowsiness or CNS effects may also occur.
    Heparin: (Minor) Antihistamines may partially counteract the anticoagulant actions of heparin, according to the product labels. However, this interaction is not likely of clinical significance since heparin therapy is adjusted to the partial thromboplastin time (aPTT) and other clinical parameters of the patient.
    Homatropine; Hydrocodone: (Moderate) Concomitant use of hydrocodone with other CNS depressants may lead to hypotension, profound sedation, coma, respiratory depression and death. Prior to concurrent use of hydrocodone in patients taking a CNS depressant, assess the level of tolerance to CNS depression that has developed, the duration of use, and the patient's overall response to treatment. Consider the patient's use of alcohol or illicit drugs. Hydrocodone should be used in reduced dosages if used concurrently with a CNS depressant; initiate hydrocodone at 20 to 30% of the usual dosage in patients that are concurrently receiving another CNS depressant. Also, consider a using a lower dose of the CNS depressant. Monitor patients for sedation and respiratory depression. In addition, the metabolism of hydrocodone to its active metabolite, hydromorphone, is dependent on CYP2D6. Theoretically, coadministration of hydrocodone and a CYP2D6 inhibitor, such as chlorpheniramine or diphenhydramine, may result in a reduction in the analgesic effect of hydrocodone.
    Hyaluronidase, Recombinant; Immune Globulin: (Minor) H1-blockers (antihistamines), when given in large systemic doses, may render tissues partially resistant to the action of hyaluronidase. Patients receiving these medications may require larger amounts of hyaluronidase for equivalent dispersing effect.
    Hyaluronidase: (Minor) H1-blockers (antihistamines), when given in large systemic doses, may render tissues partially resistant to the action of hyaluronidase. Patients receiving these medications may require larger amounts of hyaluronidase for equivalent dispersing effect.
    Hydantoins: (Moderate) Hydantoin anticonvulsants can theoretically add to the CNS depressant effects of other CNS depressants including the sedating H1 blockers.
    Hydrocodone: (Moderate) Concomitant use of hydrocodone with other CNS depressants may lead to hypotension, profound sedation, coma, respiratory depression and death. Prior to concurrent use of hydrocodone in patients taking a CNS depressant, assess the level of tolerance to CNS depression that has developed, the duration of use, and the patient's overall response to treatment. Consider the patient's use of alcohol or illicit drugs. Hydrocodone should be used in reduced dosages if used concurrently with a CNS depressant; initiate hydrocodone at 20 to 30% of the usual dosage in patients that are concurrently receiving another CNS depressant. Also, consider a using a lower dose of the CNS depressant. Monitor patients for sedation and respiratory depression. In addition, the metabolism of hydrocodone to its active metabolite, hydromorphone, is dependent on CYP2D6. Theoretically, coadministration of hydrocodone and a CYP2D6 inhibitor, such as chlorpheniramine or diphenhydramine, may result in a reduction in the analgesic effect of hydrocodone.
    Hydrocodone; Ibuprofen: (Moderate) Concomitant use of hydrocodone with other CNS depressants may lead to hypotension, profound sedation, coma, respiratory depression and death. Prior to concurrent use of hydrocodone in patients taking a CNS depressant, assess the level of tolerance to CNS depression that has developed, the duration of use, and the patient's overall response to treatment. Consider the patient's use of alcohol or illicit drugs. Hydrocodone should be used in reduced dosages if used concurrently with a CNS depressant; initiate hydrocodone at 20 to 30% of the usual dosage in patients that are concurrently receiving another CNS depressant. Also, consider a using a lower dose of the CNS depressant. Monitor patients for sedation and respiratory depression. In addition, the metabolism of hydrocodone to its active metabolite, hydromorphone, is dependent on CYP2D6. Theoretically, coadministration of hydrocodone and a CYP2D6 inhibitor, such as chlorpheniramine or diphenhydramine, may result in a reduction in the analgesic effect of hydrocodone.
    Hydrocodone; Phenylephrine: (Moderate) Concomitant use of hydrocodone with other CNS depressants may lead to hypotension, profound sedation, coma, respiratory depression and death. Prior to concurrent use of hydrocodone in patients taking a CNS depressant, assess the level of tolerance to CNS depression that has developed, the duration of use, and the patient's overall response to treatment. Consider the patient's use of alcohol or illicit drugs. Hydrocodone should be used in reduced dosages if used concurrently with a CNS depressant; initiate hydrocodone at 20 to 30% of the usual dosage in patients that are concurrently receiving another CNS depressant. Also, consider a using a lower dose of the CNS depressant. Monitor patients for sedation and respiratory depression. In addition, the metabolism of hydrocodone to its active metabolite, hydromorphone, is dependent on CYP2D6. Theoretically, coadministration of hydrocodone and a CYP2D6 inhibitor, such as chlorpheniramine or diphenhydramine, may result in a reduction in the analgesic effect of hydrocodone.
    Hydrocodone; Potassium Guaiacolsulfonate: (Moderate) Concomitant use of hydrocodone with other CNS depressants may lead to hypotension, profound sedation, coma, respiratory depression and death. Prior to concurrent use of hydrocodone in patients taking a CNS depressant, assess the level of tolerance to CNS depression that has developed, the duration of use, and the patient's overall response to treatment. Consider the patient's use of alcohol or illicit drugs. Hydrocodone should be used in reduced dosages if used concurrently with a CNS depressant; initiate hydrocodone at 20 to 30% of the usual dosage in patients that are concurrently receiving another CNS depressant. Also, consider a using a lower dose of the CNS depressant. Monitor patients for sedation and respiratory depression. In addition, the metabolism of hydrocodone to its active metabolite, hydromorphone, is dependent on CYP2D6. Theoretically, coadministration of hydrocodone and a CYP2D6 inhibitor, such as chlorpheniramine or diphenhydramine, may result in a reduction in the analgesic effect of hydrocodone.
    Hydrocodone; Potassium Guaiacolsulfonate; Pseudoephedrine: (Moderate) Concomitant use of hydrocodone with other CNS depressants may lead to hypotension, profound sedation, coma, respiratory depression and death. Prior to concurrent use of hydrocodone in patients taking a CNS depressant, assess the level of tolerance to CNS depression that has developed, the duration of use, and the patient's overall response to treatment. Consider the patient's use of alcohol or illicit drugs. Hydrocodone should be used in reduced dosages if used concurrently with a CNS depressant; initiate hydrocodone at 20 to 30% of the usual dosage in patients that are concurrently receiving another CNS depressant. Also, consider a using a lower dose of the CNS depressant. Monitor patients for sedation and respiratory depression. In addition, the metabolism of hydrocodone to its active metabolite, hydromorphone, is dependent on CYP2D6. Theoretically, coadministration of hydrocodone and a CYP2D6 inhibitor, such as chlorpheniramine or diphenhydramine, may result in a reduction in the analgesic effect of hydrocodone.
    Hydrocodone; Pseudoephedrine: (Moderate) Concomitant use of hydrocodone with other CNS depressants may lead to hypotension, profound sedation, coma, respiratory depression and death. Prior to concurrent use of hydrocodone in patients taking a CNS depressant, assess the level of tolerance to CNS depression that has developed, the duration of use, and the patient's overall response to treatment. Consider the patient's use of alcohol or illicit drugs. Hydrocodone should be used in reduced dosages if used concurrently with a CNS depressant; initiate hydrocodone at 20 to 30% of the usual dosage in patients that are concurrently receiving another CNS depressant. Also, consider a using a lower dose of the CNS depressant. Monitor patients for sedation and respiratory depression. In addition, the metabolism of hydrocodone to its active metabolite, hydromorphone, is dependent on CYP2D6. Theoretically, coadministration of hydrocodone and a CYP2D6 inhibitor, such as chlorpheniramine or diphenhydramine, may result in a reduction in the analgesic effect of hydrocodone.
    Hydromorphone: (Moderate) Concomitant use of hydromorphone with other central nervous system (CNS) depressants can potentiate the effects of hydromorphone and may lead to additive CNS or respiratory depression, profound sedation, or coma. Examples of drugs associated with CNS depression include sedating H1-blockers. Prior to concurrent use of hydromorphone in patients taking a CNS depressant, assess the level of tolerance to CNS depression that has developed, the duration of use, and the patient's overall response to treatment. Consider the patient's use of alcohol or illicit drugs. If hydromorphone is used concurrently with a CNS depressant, a reduced dosage of hydromorphone and/or the CNS depressant is recommended; start with one-third to one-half of the estimated hydromorphone starting dose when using hydromorphone extended-release tablets. Carefully monitor the patient for hypotension, CNS depression, and respiratory depression. Carbon dioxide retention from opioid-induced respiratory depression can exacerbate the sedating effects of opioids.
    Ibuprofen; Oxycodone: (Moderate) Concomitant use of oxycodone with sedating H1-blockers may lead to additive respiratory and/or CNS depression. Hypotension, profound sedation, coma, respiratory depression, or death may occur. Prior to concurrent use of oxycodone in patients taking a CNS depressant, assess the level of tolerance to CNS depression that has developed, the duration of use, and the patient's overall response to treatment. Consider the patient's use of alcohol or illicit drugs. If a CNS depressant is used concurrently with oxycodone, a reduced dosage of oxycodone and/or the CNS depressant is recommended; use an initial dose of oxycodone at 1/3 to 1/2 the usual dosage. Monitor for sedation and respiratory depression.
    Iloperidone: (Moderate) Drugs that can cause CNS depression, if used concomitantly with iloperidone, may increase both the frequency and the intensity of adverse effects such as drowsiness, sedation, and dizziness. Caution should be used when iloperidone is given in combination with other centrally-acting medications, such as sedating H1-blockers.
    Isocarboxazid: (Major) Concurrent use of monoamine oxidase inhibitors (MAOIs) and sedating H1-blockers (sedating antihistamines) may result in additive sedation, anticholinergic effects, or hypotensive reactions. Consider alternative therapy to antihistamines where possible. If alternative combinations are not available, these medications may be used together with close monitoring. Many non-prescription products for coughs, colds, allergy, hay fever or insomnia contain sedating antihistamines. Patients receiving an MAOI should be counseled that it is essential to consult their healthcare provider or pharmacist prior to the use of any non-prescription products. Patients should also be advised against driving or engaging in other activities requiring mental alertness until they know how this combination affects them.
    Ketamine: (Minor) Because sedating H1-blockers cause sedation, an enhanced CNS depressant effect may occur when they are combined with general anesthetics.
    Levocetirizine: (Major) Dry mouth, drowsiness and other antihistamine-related side effects may occur in patients receiving cetirizine. Due to the duplicative and additive nature of the pharmacology of cetirizine, concurrent use of sedating antihistamines (H1-blockers) is not recommended.
    Levomethadyl: (Moderate) Enhanced CNS depressant effects may occur when levomethadyl is combined with other CNS depressants, such as sedating H1 blockers.
    Levorphanol: (Moderate) Concomitant use of levorphanol with other CNS depressants such as sedating H1-blockers can potentiate the effects of levorphanol on respiration, blood pressure, and alertness. Severe hypotension, respiratory depression, profound sedation, or coma may occur. Prior to concurrent use of levorphanol in patients taking a CNS depressant, assess the level of tolerance to CNS depression that has developed, the duration of use, and the patient's overall response to treatment. Consider the patient's use of alcohol or illicit drugs. When concomitant treatment with levorphanol with another CNS depressant is necessary, reduce the dose of 1 or both drugs. The initial dose of levorphanol should be reduced by approximately 50% or more when levorphanol is used with another drug that may depress respiration.
    Lisdexamfetamine: (Moderate) Amphetamines may pharmacodynamically counteract the sedative properties of sedating H1-blockers. This effect may be clinically important if a patient is receiving an antihistamine agent for treatment of insomnia. Alternatively, if a patient is receiving an amphetamine for treatment of narcolepsy, the combination with a sedating antihistamine may reverse the action of the amphetamine. Coadminister with caution and monitor for altered response to drug therapy.
    Lithium: (Moderate) Because lithium has the potential to impair cognitive and motor skills, caution is advisable during concurrent use of other medications with centrally-acting effects including the sedating antihistamines.
    Loratadine: (Minor) Although loratadine is considered a 'non-sedating' antihistamine, dose-related sedation has been noted. For this reason, it would be prudent to monitor for drowsiness during concurrent use of loratadine with CNS depressants such as other H1-blockers.
    Loratadine; Pseudoephedrine: (Minor) Although loratadine is considered a 'non-sedating' antihistamine, dose-related sedation has been noted. For this reason, it would be prudent to monitor for drowsiness during concurrent use of loratadine with CNS depressants such as other H1-blockers.
    Loxapine: (Moderate) Sedating H1-blockers are associated with anticholinergic effects and sedation; therefore, additive effects may be seen during concurrent use with other drugs having anticholinergic activity and CNS depressant properties such as traditional antipsychotic agents, including loxapine. Clinicians should note that antimuscarinic effects may be seen not only on GI smooth muscle, but also on bladder function, the eye, and temperature regulation. Additive drowsiness or other CNS effects may also occur.
    Lurasidone: (Moderate) Due to the CNS effects of lurasidone, caution should be used when lurasidone is given in combination with other centrally acting medications. Sedating H1-blockers are associated with sedation; therefore, additive effects may be seen during concurrent use with other drugs having CNS depressant properties such as antipsychotics. Additive drowsiness or other CNS effects may occur.
    Magnesium Salts: (Minor) Because of the CNS-depressant effects of magnesium sulfate, additive central-depressant effects can occur following concurrent administration with CNS depressants such as sedating H1-blockers. Caution should be exercised when using these agents concurrently.
    Maprotiline: (Moderate) Additive anticholinergic effects may be seen when maprotiline is used concomitantly with other commonly used drugs with moderate to significant anticholinergic effects including sedating h1-blockers.
    Meclizine: (Major) Meclizine is an H1-blocker which exhibits significant anticholinergic effects. The anticholinergic effects of meclizine may be enhanced when combined with other drugs with antimuscarinic activity, including other sedating H1-blockers. Clinicians should note that antimuscarinic effects might be seen not only on GI smooth muscle, but also on bladder function, the eye, and temperature regulation. Additive sedation may also occur.
    Melatonin: (Moderate) Concomitant administration of sedating antihistamines and melatonin may cause additive CNS depression and should be used cautiously in combination. Especially use caution when combining melatonin with sedating antihistamines found in OTC sleep products, since over-sedation, CNS effects, or sleep-related behaviors may occur. Use of more than one agent for hypnotic purposes may increase the risk for over-sedation, CNS effects, or sleep-related behaviors. Be alert for unusual changes in moods or behaviors. Patients reporting unusual sleep-related behaviors likely should discontinue melatonin use.
    Meperidine: (Moderate) Enhanced CNS depressant effects may occur when meperidine is combined with other CNS depressants, such as sedating H1 blockers.
    Meperidine; Promethazine: (Moderate) Additive anticholinergic effects may be seen when promethazine is used concomitantly with other drugs with antimuscarinic activity like sedating H1-blockers. Clinicians should note that antimuscarinic effects may be seen not only on GI smooth muscle, but also on bladder function, the eye, and temperature regulation. Because promethazine causes pronounced sedation, an enhanced CNS depressant effect or additive drowsiness may occur when it is combined with other CNS depressants including sedating H1-blockers. (Moderate) Enhanced CNS depressant effects may occur when meperidine is combined with other CNS depressants, such as sedating H1 blockers.
    Mephobarbital: (Moderate) Additive CNS depression may occur if barbiturates are co-used with sedating antihistamines, such as clemastine. Monitor for additive CNS and respiratory effects, and warn about the potential effects to driving and other activities.
    Meprobamate: (Moderate) The CNS-depressant effects of meprobamate can be potentiated with concomitant administration of other drugs known to cause CNS depression including sedating H1-blockers.
    Mesoridazine: (Moderate) Additive effects may be seen when phenothiazines are used concomitantly with other drugs with antimuscarinic activity, such as clemastine, a sedating H1-blocker. Phenothiazines, such as thioridazine and chlorpromazine, may exhibit pronounced anticholinergic and sedative effects. Clinicians should note that antimuscarinic effects may be seen not only on GI smooth muscle, but also on bladder function, the eye, and temperature regulation. Additive drowsiness or other CNS effects may also occur.
    Metaxalone: (Moderate) Concomitant administration of metaxalone with other CNS depressants can potentiate the sedative effects of either agent.
    Methadone: (Moderate) Concomitant use of methadone with another CNS depressant can lead to additive respiratory depression, hypotension, profound sedation, or coma; examples include sedating H1-blockers. Prior to concurrent use of methadone in patients taking a CNS depressant, assess the level of tolerance to CNS depression that has developed, the duration of use, and the patient's overall response to treatment. Consider the patient's use of alcohol or illicit drugs. Methadone should be used with caution and in reduced dosages if used concurrently with a CNS depressant; in opioid-naive adults, use an initial methadone dose of 2.5 mg every 12 hours. Also consider a using a lower dose of the CNS depressant. Monitor patients for sedation and respiratory depression.
    Methamphetamine: (Moderate) Amphetamines may pharmacodynamically counteract the sedative properties of sedating H1-blockers. This effect may be clinically important if a patient is receiving an antihistamine agent for treatment of insomnia. Alternatively, if a patient is receiving an amphetamine for treatment of narcolepsy, the combination with a sedating antihistamine may reverse the action of the amphetamine. Coadminister with caution and monitor for altered response to drug therapy.
    Methocarbamol: (Moderate) Methocarbamol may cause additive CNS depression if used concomitantly with other CNS depressants such as sedating H1-blockers. Combination therapy can cause additive effects of sedation and dizziness, which can impair the patient's ability to undertake tasks requiring mental alertness. Dosage adjustments of either or both medications may be necessary.
    Methohexital: (Moderate) Additive CNS depression may occur if barbiturates are co-used with sedating antihistamines, such as clemastine. Monitor for additive CNS and respiratory effects, and warn about the potential effects to driving and other activities.
    Metoclopramide: (Minor) Combined use of metoclopramide and other CNS depressants, such as anxiolytics, sedatives, and hypnotics, can increase possible sedation.
    Metyrapone: (Moderate) Metyrapone may cause dizziness and/or drowsiness. Other drugs that may also cause drowsiness, such as sedating H1-blockers, should be used with caution. Additive drowsiness and/or dizziness is possible.
    Metyrosine: (Moderate) The concomitant administration of metyrosine with sedating H1-blockers can result in additive sedative effects.
    Minocycline: (Minor) Injectable minocycline contains magnesium sulfate heptahydrate. Because of the CNS-depressant effects of magnesium sulfate, additive central-depressant effects can occur following concurrent administration with CNS depressants, such as sedating H1-blockers. Caution should be exercised when using these agents concurrently.
    Mirtazapine: (Moderate) Consistent with the CNS depressant effects of mirtazapine, additive effects may occur with other CNS depressants such as clemastine. Mirtazapine should be administered cautiously with such agents because the CNS effects on cognitive performance and motor skills can be additive.
    Mitotane: (Moderate) Mitotane can cause sedation, lethargy, vertigo, and other CNS side effects. Concomitant administration of mitotane and CNS depressants, including sedating h1-blockers, may cause additive CNS effects.
    Mivacurium: (Moderate) An enhanced CNS depressant effect may occur when sedating H1-blockers are combined with other CNS depressants including neuromuscular blockers.
    Molindone: (Moderate) An enhanced CNS depressant effect may occur when sedating h1-blockers are combined with other CNS depressants including molindone.
    Monoamine oxidase inhibitors: (Major) Concurrent use of monoamine oxidase inhibitors (MAOIs) and sedating H1-blockers (sedating antihistamines) may result in additive sedation, anticholinergic effects, or hypotensive reactions. Consider alternative therapy to antihistamines where possible. If alternative combinations are not available, these medications may be used together with close monitoring. Many non-prescription products for coughs, colds, allergy, hay fever or insomnia contain sedating antihistamines. Patients receiving an MAOI should be counseled that it is essential to consult their healthcare provider or pharmacist prior to the use of any non-prescription products. Patients should also be advised against driving or engaging in other activities requiring mental alertness until they know how this combination affects them.
    Morphine: (Moderate) Concomitant use of morphine with other CNS depressants can potentiate the effects of morphine on respiration, blood pressure, and alertness; examples of other CNS depressants include sedating H1-blockers. Prior to concurrent use of morphine in patients taking a CNS depressant, assess the level of tolerance to CNS depression that has developed, the duration of use, and the patient's overall response to treatment. Consider the patient's use of alcohol or illicit drugs. If a CNS depressant is used concurrently with morphine, a reduced dosage of morphine and/or the CNS depressant is recommended; for extended-release products, start with the lowest possible dose of morphine (i.e., 15 mg PO every 12 hours, extended-release tablets; 30 mg or less PO every 24 hours, extended-release capsules). Monitor patients for sedation and respiratory depression.
    Morphine; Naltrexone: (Moderate) Concomitant use of morphine with other CNS depressants can potentiate the effects of morphine on respiration, blood pressure, and alertness; examples of other CNS depressants include sedating H1-blockers. Prior to concurrent use of morphine in patients taking a CNS depressant, assess the level of tolerance to CNS depression that has developed, the duration of use, and the patient's overall response to treatment. Consider the patient's use of alcohol or illicit drugs. If a CNS depressant is used concurrently with morphine, a reduced dosage of morphine and/or the CNS depressant is recommended; for extended-release products, start with the lowest possible dose of morphine (i.e., 15 mg PO every 12 hours, extended-release tablets; 30 mg or less PO every 24 hours, extended-release capsules). Monitor patients for sedation and respiratory depression.
    Nabilone: (Moderate) Concomitant use of nabilone with other CNS depressants, such as sedating H1-blockers, can potentiate the effects of nabilone on respiratory depression.
    Nalbuphine: (Moderate) Concomitant use of nalbuphine with other CNS depressants, such as sedating H1-blockers, can potentiate the effects of nalbuphine on respiratory depression, CNS depression, and sedation.
    Nefazodone: (Moderate) An enhanced CNS depressant effect may occur when sedating H1-blockers are combined with other CNS depressants including nefazodone.
    Neuromuscular blockers: (Moderate) An enhanced CNS depressant effect may occur when sedating H1-blockers are combined with other CNS depressants including neuromuscular blockers.
    Olanzapine: (Moderate) Olanzapine exhibits anticholinergic effects that may be clinically significant. Clinicians should keep this in mind when using antimuscarinics and other medications with anticholinergic activity in combination with olanzapine. Some medications exhibit additive anticholinergic effects include sedating H1-blockers. Olanzapine may also cause additive sedation with many of these drugs.
    Orphenadrine: (Moderate) Orphenadrine has mild anticholinergic activity. Depending on the specific agent, additive anticholinergic effects may be seen when orphenadrine is used concomitantly with sedating H1-blockers.
    Oxycodone: (Moderate) Concomitant use of oxycodone with sedating H1-blockers may lead to additive respiratory and/or CNS depression. Hypotension, profound sedation, coma, respiratory depression, or death may occur. Prior to concurrent use of oxycodone in patients taking a CNS depressant, assess the level of tolerance to CNS depression that has developed, the duration of use, and the patient's overall response to treatment. Consider the patient's use of alcohol or illicit drugs. If a CNS depressant is used concurrently with oxycodone, a reduced dosage of oxycodone and/or the CNS depressant is recommended; use an initial dose of oxycodone at 1/3 to 1/2 the usual dosage. Monitor for sedation and respiratory depression.
    Oxymorphone: (Moderate) Concomitant use of oxymorphone with other CNS depressants may produce additive CNS depressant effects. Hypotension, profound sedation, coma, respiratory depression, or death may occur; examples of other CNS depressants include sedating H1-blockers. Prior to concurrent use of oxymorphone in patients taking a CNS depressant, assess the level of tolerance to CNS depression that has developed, the duration of use, and the patient's overall response to treatment. Consider the patient's use of alcohol or illicit drugs. If a CNS depressant is used concurrently with oxymorphone, a reduced dosage of oxymorphone (1/3 to 1/2 of the usual dose) and/or the CNS depressant is recommended. If the extended-release oxymorphone tablets are used concurrently with a CNS depressant, it is recommended to use an initial dosage of 5 mg PO every 12 hours. Monitor for sedation or respiratory depression.
    Paliperidone: (Moderate) Drugs that can cause CNS depression, if used concomitantly with paliperidone, can increase both the frequency and the intensity of adverse effects such as drowsiness, sedation, and dizziness. Caution should be used when paliperidone is given in combination with other centrally-acting medications including sedating H1-blockers.
    Pancuronium: (Moderate) An enhanced CNS depressant effect may occur when sedating H1-blockers are combined with other CNS depressants including neuromuscular blockers.
    Pentazocine: (Moderate) Use pentazocine with caution in any patient receiving medication with CNS depressant and/or anticholinergic activity. Coadministration of pentazocine with sedating H1-blockers may result in additive respiratory and CNS depression and anticholinergic effects, such as urinary retention and constipation.
    Pentazocine; Naloxone: (Moderate) Use pentazocine with caution in any patient receiving medication with CNS depressant and/or anticholinergic activity. Coadministration of pentazocine with sedating H1-blockers may result in additive respiratory and CNS depression and anticholinergic effects, such as urinary retention and constipation.
    Pentobarbital: (Moderate) Additive CNS depression may occur if barbiturates are co-used with sedating antihistamines, such as clemastine. Monitor for additive CNS and respiratory effects, and warn about the potential effects to driving and other activities.
    Perampanel: (Moderate) Co-administration of perampanel with CNS depressants, including ethanol, may increase CNS depression. The combination of perampanel (particularly at high doses) with ethanol has led to decreased mental alertness and ability to perform complex tasks (such as driving), as well as increased levels of anger, confusion, and depression; similar reactions should be expected with concomitant use of other CNS depressants, such as sedating H1-blockers.
    Perphenazine: (Moderate) Additive effects may be seen when phenothiazines are used concomitantly with other drugs with antimuscarinic activity, such as clemastine, a sedating H1-blocker. Phenothiazines, such as thioridazine and chlorpromazine, may exhibit pronounced anticholinergic and sedative effects. Clinicians should note that antimuscarinic effects may be seen not only on GI smooth muscle, but also on bladder function, the eye, and temperature regulation. Additive drowsiness or other CNS effects may also occur.
    Perphenazine; Amitriptyline: (Moderate) Additive effects may be seen when phenothiazines are used concomitantly with other drugs with antimuscarinic activity, such as clemastine, a sedating H1-blocker. Phenothiazines, such as thioridazine and chlorpromazine, may exhibit pronounced anticholinergic and sedative effects. Clinicians should note that antimuscarinic effects may be seen not only on GI smooth muscle, but also on bladder function, the eye, and temperature regulation. Additive drowsiness or other CNS effects may also occur.
    Phenelzine: (Major) Concurrent use of monoamine oxidase inhibitors (MAOIs) and sedating H1-blockers (sedating antihistamines) may result in additive sedation, anticholinergic effects, or hypotensive reactions. Consider alternative therapy to antihistamines where possible. If alternative combinations are not available, these medications may be used together with close monitoring. Many non-prescription products for coughs, colds, allergy, hay fever or insomnia contain sedating antihistamines. Patients receiving an MAOI should be counseled that it is essential to consult their healthcare provider or pharmacist prior to the use of any non-prescription products. Patients should also be advised against driving or engaging in other activities requiring mental alertness until they know how this combination affects them.
    Phenobarbital: (Moderate) Additive CNS depression may occur if barbiturates are co-used with sedating antihistamines, such as clemastine. Monitor for additive CNS and respiratory effects, and warn about the potential effects to driving and other activities.
    Phenothiazines: (Moderate) Additive effects may be seen when phenothiazines are used concomitantly with other drugs with antimuscarinic activity, such as clemastine, a sedating H1-blocker. Phenothiazines, such as thioridazine and chlorpromazine, may exhibit pronounced anticholinergic and sedative effects. Clinicians should note that antimuscarinic effects may be seen not only on GI smooth muscle, but also on bladder function, the eye, and temperature regulation. Additive drowsiness or other CNS effects may also occur.
    Phenylephrine; Promethazine: (Moderate) Additive anticholinergic effects may be seen when promethazine is used concomitantly with other drugs with antimuscarinic activity like sedating H1-blockers. Clinicians should note that antimuscarinic effects may be seen not only on GI smooth muscle, but also on bladder function, the eye, and temperature regulation. Because promethazine causes pronounced sedation, an enhanced CNS depressant effect or additive drowsiness may occur when it is combined with other CNS depressants including sedating H1-blockers.
    Pimozide: (Moderate) Due to the effects of pimozide on cognition, it should be used cautiously with other CNS depressants including sedating antihistamines. Sedating H1-blockers are associated with anticholinergic effects and sedation; therefore, additive effects may be seen during concurrent use with pimozide. Additive drowsiness or other CNS effects may occur.
    Pramipexole: (Moderate) Concomitant use of pramipexole with other CNS depressants, such as sedating H1-blockers, can potentiate the sedation effects of pramipexole.
    Pregabalin: (Moderate) Concomitant administration of pregabalin with CNS-depressant drugs, including sedating H1-blockers, can potentiate the CNS effects of either agent. Pregabalin can cause considerable somnolence and the combined use of ethanol or other CNS depressants with pregabalin may lead to an additive drowsy effect.
    Primidone: (Moderate) Additive CNS depression may occur if barbiturates are co-used with sedating antihistamines, such as clemastine. Monitor for additive CNS and respiratory effects, and warn about the potential effects to driving and other activities.
    Procarbazine: (Moderate) Use procarbazine and sedating H1-blockers together with caution; additive central nervous system depression may occur.
    Prochlorperazine: (Moderate) Additive effects may be seen when phenothiazines are used concomitantly with other drugs with antimuscarinic activity, such as clemastine, a sedating H1-blocker. Phenothiazines, such as thioridazine and chlorpromazine, may exhibit pronounced anticholinergic and sedative effects. Clinicians should note that antimuscarinic effects may be seen not only on GI smooth muscle, but also on bladder function, the eye, and temperature regulation. Additive drowsiness or other CNS effects may also occur.
    Promethazine: (Moderate) Additive anticholinergic effects may be seen when promethazine is used concomitantly with other drugs with antimuscarinic activity like sedating H1-blockers. Clinicians should note that antimuscarinic effects may be seen not only on GI smooth muscle, but also on bladder function, the eye, and temperature regulation. Because promethazine causes pronounced sedation, an enhanced CNS depressant effect or additive drowsiness may occur when it is combined with other CNS depressants including sedating H1-blockers.
    Propofol: (Minor) Because sedating H1-blockers cause sedation, an enhanced CNS depressant effect may occur when they are combined with general anesthetics.
    Propoxyphene: (Moderate) Concomitant use of propoxyphene with other CNS depressants can potentiate respiratory depression and, or sedation. In addition, chlorpheniramine and diphenhydramine inhibit CYP2D6, an enzyme responsible for the metabolism of propoxyphene. Monitor these patients. Overdosage of propoxyphene in combination with other potent CNS depressants is a major cause of drug-related death; fatalities within the first hour of overdosage are not uncommon.
    Quetiapine: (Moderate) Somnolence is a commonly reported adverse effect of quetiapine. Co-administration of quetiapine with sedating H1-blockers may result in additive effects. Additive drowsiness or other CNS effects may occur.
    Ramelteon: (Moderate) Because sedating H1-blockers cause sedation, an enhanced CNS depressant effect may occur when it is combined with other CNS depressants including anxiolytics, sedatives, and hypnotics, such as ramelteon.
    Rapacuronium: (Moderate) An enhanced CNS depressant effect may occur when sedating H1-blockers are combined with other CNS depressants including neuromuscular blockers.
    Rasagiline: (Moderate) Concurrent use of monoamine oxidase inhibitors (MAOIs) and sedating H1-blockers (sedating antihistamines) may result in additive sedation, anticholinergic effects, or hypotensive reactions. Rasagiline may be less likely to produce these interactions than other MAOIs, due to MAO-B selectivity. However, consider alternatives therapy to antihistamines where possible. If alternative combinations are not available, these medications may be used together with close monitoring. Many non-prescription products for coughs, colds, allergy, hay fever or insomnia contain sedating antihistamines. Patients receiving rasagiline should be counseled that it is essential to consult their healthcare provider or pharmacist prior to the use of any non-prescription products. Patients should also be advised against driving or engaging in other activities requiring mental alertness until they know how this combination affects them.
    Remifentanil: (Moderate) Concomitant use of remifentanil with other CNS depressants can potentiate the effects of remifentanil on respiration, sedation, and hypotension. A dose reduction of one or both drugs may be warranted.
    Risperidone: (Moderate) Due to the primary CNS effects of risperidone, caution should be used when risperidone is given in combination with other centrally acting medications including sedating H1-blockers. Additive drowsiness or other CNS effects may occur.
    Rituximab; Hyaluronidase: (Minor) H1-blockers (antihistamines), when given in large systemic doses, may render tissues partially resistant to the action of hyaluronidase. Patients receiving these medications may require larger amounts of hyaluronidase for equivalent dispersing effect.
    Rivastigmine: (Moderate) Concurrent use of sedating H1-blockers and rivastigmine should be avoided if possible. Rivastigmine inhibits acetylcholinesterase, the enzyme responsible for the degradation of acetylcholine, and improves the availability of acetylcholine. Sedating H1-blockers may exhibit significant anticholinergic activity, thereby interfering with the therapeutic effect of rivastigmine.
    Rocuronium: (Moderate) An enhanced CNS depressant effect may occur when sedating H1-blockers are combined with other CNS depressants including neuromuscular blockers.
    Ropinirole: (Moderate) Concomitant use of ropinirole with other CNS depressants, such as sedating H1-blockers, can potentiate the sedation effects of ropinirole.
    Safinamide: (Moderate) Dopaminergic medications, including safinamide, may cause a sudden onset of somnolence which sometimes has resulted in motor vehicle accidents. Patients may not perceive warning signs, such as excessive drowsiness, or they may report feeling alert immediately prior to the event. Because of possible additive effects, advise patients about the potential for increased somnolence during concurrent use of other sedating medications, such as sedating H1-blockers.
    Secobarbital: (Moderate) Additive CNS depression may occur if barbiturates are co-used with sedating antihistamines, such as clemastine. Monitor for additive CNS and respiratory effects, and warn about the potential effects to driving and other activities.
    Selegiline: (Major) Concurrent use of monoamine oxidase inhibitors (MAOIs) and sedating H1-blockers (sedating antihistamines) may result in additive sedation, anticholinergic effects, or hypotensive reactions. Consider alternative therapy to antihistamines where possible. If alternative combinations are not available, these medications may be used together with close monitoring. Many non-prescription products for coughs, colds, allergy, hay fever or insomnia contain sedating antihistamines. Patients receiving an MAOI should be counseled that it is essential to consult their healthcare provider or pharmacist prior to the use of any non-prescription products. Patients should also be advised against driving or engaging in other activities requiring mental alertness until they know how this combination affects them.
    Sincalide: (Moderate) Sincalide-induced gallbladder ejection fraction may be affected by concurrent medications, including H1-blockers. False study results are possible; thorough patient history is important in the interpretation of procedure results.
    Sodium Iodide: (Moderate) Antihistamines may alter sodium iodide I-131 pharmacokinetics and dynamics for up to 1 week after administration. In addition, medications that decrease salivation increase the time of radiation exposure to salivary glands. Consider discontinuing sedating H1-blockers prior to sodium iodide I-131 administration.
    Solifenacin: (Moderate) Depending on the specific agent, additive anticholinergic effects may be seen when drugs with antimuscarinic properties like solifenacin are used concomitantly with other antimuscarinics, such as sedating H1 blockers.
    Succinylcholine: (Moderate) An enhanced CNS depressant effect may occur when sedating H1-blockers are combined with other CNS depressants including neuromuscular blockers.
    Sufentanil: (Moderate) Concomitant use of sufentanil with other CNS depressants can potentiate sufentanil-induced CNS and cardiovascular effects and the duration of these effects. A dose reduction of one or both drugs may be warranted.
    Suvorexant: (Moderate) CNS depressant drugs may have cumulative effects when administered concurrently and they should be used cautiously with suvorexant. A reduction in dose of the CNS depressant may be needed in some cases.
    Tacrine: (Moderate) Concurrent use of sedating H1-blockers and tacrine should be avoided if possible. Tacrine inhibits acetylcholinesterase, the enzyme responsible for the degradation of acetylcholine, and improves the availability of acetylcholine. Sedating H1-blockers may exhibit significant anticholinergic activity, thereby interfering with the therapeutic effect of tacrine.
    Tapentadol: (Moderate) Additive CNS depressive effects are expected if tapentadol is used in conjunction with other CNS depressants. Severe hypotension, profound sedation, coma, or respiratory depression may occur. Prior to concurrent use of tapentadol in patients taking a CNS depressant, assess the level of tolerance to CNS depression that has developed, the duration of use, and the patient's overall response to treatment. Consider the patient's use of alcohol or illicit drugs. If a CNS depressant is used concurrently with tapentadol, a reduced dosage of tapentadol and/or the CNS depressant is recommended. If the extended-release tapentadol tablets are used concurrently with a CNS depressant, it is recommended to use an initial tapentadol dose of 50 mg PO every 12 hours. Monitor patients for sedation and respiratory depression.
    Tasimelteon: (Moderate) Because sedating H1-blockers cause sedation, an enhanced CNS depressant effect may occur when it is combined with other CNS depressants including anxiolytics, sedatives, and hypnotics, such as tasimelteon.
    Thalidomide: (Major) Avoid the concomitant use of thalidomide with opiate agonists; antihistamines; antipsychotics; anxiolytics, sedatives, and hypnotics; and other central nervous system depressants due to the potential for additive sedative effects.
    Thiethylperazine: (Moderate) Additive effects may be seen when phenothiazines are used concomitantly with other drugs with antimuscarinic activity, such as clemastine, a sedating H1-blocker. Phenothiazines, such as thioridazine and chlorpromazine, may exhibit pronounced anticholinergic and sedative effects. Clinicians should note that antimuscarinic effects may be seen not only on GI smooth muscle, but also on bladder function, the eye, and temperature regulation. Additive drowsiness or other CNS effects may also occur.
    Thiopental: (Moderate) Additive CNS depression may occur if barbiturates are co-used with sedating antihistamines, such as clemastine. Monitor for additive CNS and respiratory effects, and warn about the potential effects to driving and other activities.
    Thioridazine: (Moderate) Additive effects may be seen when phenothiazines are used concomitantly with other drugs with antimuscarinic activity, such as clemastine, a sedating H1-blocker. Phenothiazines, such as thioridazine and chlorpromazine, may exhibit pronounced anticholinergic and sedative effects. Clinicians should note that antimuscarinic effects may be seen not only on GI smooth muscle, but also on bladder function, the eye, and temperature regulation. Additive drowsiness or other CNS effects may also occur.
    Thiothixene: (Moderate) Additive anticholinergic effects may be seen when antipsychotics, such as thiothixene, are used concomitantly with other drugs such as sedating H1-blockers. Additive drowsiness or other CNS effects may also occur.
    Tizanidine: (Moderate) Concurrent use of tizanidine and CNS depressants like sedating h1-blockers can cause additive CNS depression.
    Tolcapone: (Moderate) COMT inhibitors, such as entacapone or tolcapone, should be given cautiously with other agents that cause CNS depression, including sedating H1-blockers, due to the possibility of additive sedation.
    Tramadol: (Moderate) An enhanced CNS depressant effect may occur when sedating h1-blockers are combined with other CNS depressants including tramadol.
    Tranylcypromine: (Major) Concurrent use of monoamine oxidase inhibitors (MAOIs) and sedating H1-blockers (sedating antihistamines) may result in additive sedation, anticholinergic effects, or hypotensive reactions. Consider alternative therapy to antihistamines where possible. If alternative combinations are not available, these medications may be used together with close monitoring. Many non-prescription products for coughs, colds, allergy, hay fever or insomnia contain sedating antihistamines. Patients receiving an MAOI should be counseled that it is essential to consult their healthcare provider or pharmacist prior to the use of any non-prescription products. Patients should also be advised against driving or engaging in other activities requiring mental alertness until they know how this combination affects them.
    Trazodone: (Moderate) CNS depressants should be used cautiously in patients receiving trazodone because of additive CNS-depressant effects, including possible respiratory depression or hypotension.
    Tricyclic antidepressants: (Moderate) Additive anticholinergic and CNS effects may be seen when tricyclic antidepressants are used concomitantly with sedating H1-blockers. Clinicians should note that antimuscarinic effects might be seen not only on GI smooth muscle, but also on bladder function, the eye, and temperature regulation.
    Trifluoperazine: (Moderate) Additive effects may be seen when phenothiazines are used concomitantly with other drugs with antimuscarinic activity, such as clemastine, a sedating H1-blocker. Phenothiazines, such as thioridazine and chlorpromazine, may exhibit pronounced anticholinergic and sedative effects. Clinicians should note that antimuscarinic effects may be seen not only on GI smooth muscle, but also on bladder function, the eye, and temperature regulation. Additive drowsiness or other CNS effects may also occur.
    Trimethobenzamide: (Moderate) The concurrent use of trimethobenzamide with other medications that cause CNS depression, like the sedating h1-blockers, may potentiate the effects of either trimethobenzamide or the sedating h1-blocker.
    Trospium: (Moderate) Additive anticholinergic effects may be seen when trospium is used concomitantly with drugs that are known to possess relatively significant antimuscarinic properties, including sedating H1-blockers. Clinicians should note that additive antimuscarinic effects may be seen not only on GI smooth muscle, but also on bladder function and temperature regulation. While CNS-related side effects such as drowsiness and blurred vision are not typically noted with trospium, they may occur in some patients.
    Tubocurarine: (Moderate) An enhanced CNS depressant effect may occur when sedating H1-blockers are combined with other CNS depressants including neuromuscular blockers.
    Vecuronium: (Moderate) An enhanced CNS depressant effect may occur when sedating H1-blockers are combined with other CNS depressants including neuromuscular blockers.
    Vigabatrin: (Moderate) Vigabatrin may cause somnolence and fatigue. Drugs that can cause CNS depression, if used concomitantly with vigabatrin, may increase both the frequency and the intensity of adverse effects such as drowsiness, sedation, and dizziness. Caution should be used when vigabatrin is given with sedating H1-blockers.
    Vilazodone: (Moderate) Due to the CNS effects of vilazodone, caution should be used when vilazodone is given in combination with other centrally acting medications such as anxiolytics, sedatives, and hypnotics. Also, Cyproheptadine is an antagonist of serotonin in the CNS, a property which may oppose some of the pharmacologic effects of vilazodone. Cyproheptadine has been used for the management of orgasm dysfunction caused by the serotonergic antidepressants and for the adjunctive treatment of serotonin syndrome; however, a reversal of antidepressant effects may occur when cyproheptadine is given in a routine manner along with the antidepressant. Clinically, cyproheptadine reportedly has interfered with the antidepressant and anti-bulimia actions of fluoxetine, but more data are needed to confirm a direct drug-drug interaction.
    Zaleplon: (Moderate) In premarketing studies, zaleplon potentiated the CNS effects of ethanol, imipramine, and thioridazine for at least 2 to 4 hours. Other drugs that may have additive CNS effects with zaleplon but have not been studied include other sedating H1-blockers. If used together, a reduction in the dose of one or both drugs may be needed.
    Ziconotide: (Moderate) Sedating H1-blockers are CNS depressant medications that may increase drowsiness, dizziness, and confusion that are associated with ziconotide.
    Ziprasidone: (Moderate) Sedating H1-blockers are associated with sedation; therefore, additive effects may be seen during concurrent use with other drugs having CNS depressant properties such as antipsychotics. Additive drowsiness or other CNS effects may occur with ziprasidone.
    Zolpidem: (Moderate) The CNS-depressant effects of zolpidem can be potentiated with concomitant administration of other drugs known to cause CNS depression, such as sedating H1-blockers. A dose reduction of either or both drugs should be considered to minimize additive sedative effects. For Intermezzo brand of sublingual zolpidem tablets, reduce the dose to 1.75 mg/night. The risk of next-day psychomotor impairment is increased during co-administration, which may decrease the ability to perform tasks requiring full mental alertness such as driving. In addition, sleep-related behaviors, such as sleep-driving, are more likely to occur during concurrent use of zolpidem and other CNS depressants than with zolpidem alone.

    PREGNANCY AND LACTATION

    Pregnancy

    Clemastine is classified as FDA pregnancy risk category B. Oral reproduction studies performed with clemastine fumarate in rats and rabbits at doses up to 312 and 188 times the adult human doses respectively, have revealed no evidence of teratogenic effects; however adequate human data are lacking. Most antihistamines generally are not recommended for use in pregnancy, especially during the third trimester, because of a seizure risk to the fetus. Additionally, antihistamines should be avoided in the last 2 weeks of pregnancy due to a possible association between these drugs and retrolental fibroplasia in premature neonates. Other antihistamines would be preferred to clemastine if an antihistamine is necessary for use. If an antihistamine is required in pregnancy, use of an agent with more data (e.g., loratadine) in human pregnancy is recommended by the British Society for Allergy and Clinical Immunology. Use should be limited to the lowest dose, and limited to short-term, 'as needed' administration under the prescription of a qualified health-care professional. Non-pharmacologic methods (e.g., fluids, rest) are recommended to be tried first for symptomatic relief of colds or allergies during pregnancy.

    According to the manufacturer, clemastine is contraindicated for use during breast-feeding because antihistamines can induce hyperexcitability in the infant and seizures in premature infants. In one telephone follow-up study, mothers reported irritability and colicky symptoms in 10% of infants exposed to various antihistamines and drowsiness was reported in 1.6% of infants. No reactions were reported as requiring medical attention. Antihistamines can lower basal prolactin secretion and may interfere with the establishment of lactation. Non-drug methods or, if medication is needed, nonsedating antihistamines are preferred alternatives. The British Society for Allergy and Clinical Immunology recommends cetirizine at the lowest dose as a preferred antihistamine in breast-feeding women. Loratadine is also usually considered compatible with breast-feeding. Consider the benefits of breast-feeding, the risk of potential infant drug exposure, and the risk of an untreated or inadequately treated condition. If a breast-feeding infant experiences an adverse effect related to a maternally ingested drug, healthcare providers are encouraged to report the adverse effect to the FDA.

    MECHANISM OF ACTION

    H1-antagonists do not prevent the release of histamine, as do cromolyn and nedocromil, but rather compete with free histamine for binding at H1-receptor sites. These drugs competitively antagonize the effects of histamine on H1-receptors in the GI tract, uterus, large blood vessels, and bronchial smooth muscle. Blockade of H1-receptors also suppresses the formation of edema, flare, and pruritus that result from histaminic activity.
     
    H1-antagonists also possess anticholinergic properties in varying degrees; ethanolamine derivatives such as clemastine are more potent anticholinergics than are other antihistamines. This action appears to be due to a central antimuscarinic effect, which also may account for clemastine's antiemetic effects, although the exact mechanism is unknown. Sedative effects from clemastine are a result of antagonism at central histaminergic receptors, although sedation is not as pronounced with clemastine as with other H1-antagonists in its class such as diphenhydramine. Following prolonged administration, tolerance may occur, but this can be beneficial due to a reduction in sedative effects.

    PHARMACOKINETICS

    Clemastine is administered orally. The peak effect occurs within 5 to 7 hours. The duration of action is between 10—12 hours but may persist up to 24 hours in some patients. The distribution of clemastine has not been fully determined, but the drug crosses the placenta and is excreted into breast milk. Clemastine is extensively metabolized. The terminal elimination half-life range is 4—6 hours. Clemastine and its metabolites are excreted primarily in the urine.

    Oral Route

    In general, antihistamines are well absorbed from the GI tract, but they have variable solubility, which ultimately affects the onset of action. Less soluble H1-antagonists have a slower onset of action and are less likely to cause toxicity. Onset of action of clemastine is about 15—30 minutes, with peak concentration occurring in about 2—5 hours and peak effect within 5—7 hours.