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  • CLASSES

    Osmotically-Acting Laxatives

    DEA CLASS

    Rx

    DESCRIPTION

    Synthetic derivative of lactose; a disaccharide sugar containing one molecule of galactose and one molecule of fructose; used for hepatic encephalopathy and as a laxative.

    COMMON BRAND NAMES

    Acilac, Cephulac, Cholac, Chronulac, Constilac, Constulose, Enulose, Generlac, Kristalose

    HOW SUPPLIED

    Acilac/Cephulac/Cholac/Chronulac/Constilac/Constulose/Enulose/Generlac/Lactulose Oral Sol: 10g, 15mL
    Acilac/Cephulac/Cholac/Chronulac/Constilac/Constulose/Enulose/Generlac/Lactulose Rectal Sol: 10g, 15mL
    Kristalose/Lactulose Oral Pwd F/Recon: 10g, 20g

    DOSAGE & INDICATIONS

    For the treatment of hepatic encephalopathy (portal-systemic encephalopathy, PSE).
    Oral dosage
    Adults

    30 to 45 mL (20 to 30 g) PO 3 to 4 times daily. If necessary, 30 to 45 mL PO every 1 hour may be given until a laxative effect is induced. Adjust dosage every 1 to 2 days to produce 2 to 3 soft stools daily.

    Children and Adolescents

    40 to 90 mL/day (27 to 60 g) PO given in 3 to 4 divided doses. Adjust dosage every 1 to 2 days to produce 2 to 3 soft stools daily. If the initial dose produces diarrhea, reduce the dosage. If diarrhea persists, discontinue lactulose.

    Infants

    2.5 to 10 mL/day (1.67 to 6.67 g) PO given in 3 to 4 divided doses. Adjust dosage every 1 to 2 days to produce 2 to 3 soft stools daily. If the initial dose produces diarrhea, reduce the dosage. If diarrhea persists, discontinue lactulose.

    Rectal dosage (for use in coma or impending coma)
    Adults

    300 mL diluted with 700 mL of water or normal saline rectally and retain for 30 to 60 minutes. May repeat every 4 to 6 hours as needed. If the enema is evacuated too promptly, it may be repeated. Oral therapy should replace rectal as soon as possible, with the first oral dose being given before cessation of enema.

    For the treatment of constipation, including constipation related to barium retention.
    Oral dosage (oral solution)
    Adults

    Initially, 15 to 30 mL PO once daily, increasing to 60 mL PO once daily if needed. Response may take 24 to 48 hours.

    Geriatric†

    15 to 30 mL PO once daily has been used. When lactulose was used to restore bowel movements in hemorrhoidectomy patients, 15 mL PO was given twice daily starting the day before surgery and continued for 5 days postoperatively. To induce bowel evacuation in patients with colonic retention of barium, 5 to 10 mL PO twice daily for 1 to 4 weeks has been used.

    Children†

     1 to 2 g/kg/day (1.5 to 3 mL/kg/day) PO divided once or twice daily.

    Oral dosage (powder for oral solution)
    Adults

    10 to 20 g PO daily, may increase up to 40 g/day PO. To administer, dissolve dose (10 to 20 g) in 4 ounces of water. Response may take 24 to 48 hours.

    MAXIMUM DOSAGE

    Specific maximum dosage information is not available. Individualize dosage depending on route, indication, and frequency of bowel movements.

    DOSING CONSIDERATIONS

    Hepatic Impairment

    No specific dosage adjustments are needed; adjust dosage according to clinical response and indication for use.

    Renal Impairment

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

    ADMINISTRATION

    NOTE: 15 mL of lactulose syrup contains 10 g of lactulose.
     

    Oral Administration
    Oral Liquid Formulations

    Oral solution or syrup: To improve flavor, mix lactulose with a full glass of water, milk, fruit juice, or carbonated citrus beverage. May be administered on an empty stomach for more rapid results.
    Powder for oral solution: Dissolve the contents of each packet (10 g or 20 g packet) in at least 4 ounces of water.

    Rectal Administration

    Mix 300 mL lactulose with 700 mL of water or 0.9% sodium chloride injection.
    Administer solution rectally using a rectal balloon catheter. Enema should be retained for 30—60 minutes. May be readministered if inadvertently expelled.

    STORAGE

    Acilac:
    - Dispense in original container or USP equivalent tight container
    - Do not freeze
    - Store at controlled room temperature (between 68 and 77 degrees F)
    Cephulac:
    - Do not freeze
    - Store between 36 to 86 degrees F
    Cholac:
    - Dispense in original container or USP equivalent tight container
    - Do not freeze
    - Store at controlled room temperature (between 68 and 77 degrees F)
    Chronulac:
    - Dispense in original container or USP equivalent tight container
    - Do not freeze
    - Store at controlled room temperature (between 68 and 77 degrees F)
    Constilac:
    - Dispense in original container or USP equivalent tight container
    - Do not freeze
    - Store at controlled room temperature (between 68 and 77 degrees F)
    Constulose:
    - Dispense in original container or USP equivalent tight container
    - Do not freeze
    - Store at controlled room temperature (between 68 and 77 degrees F)
    Enulose:
    - Do not freeze
    - Store between 36 to 86 degrees F
    Generlac:
    - Do not freeze
    - Store between 36 to 86 degrees F
    Kristalose :
    - Store at room temperature (between 59 to 86 degrees F)

    CONTRAINDICATIONS / PRECAUTIONS

    Galactose-free diet

    Lactulose contains galactose and is contraindicated in patients who require a galactose-free diet.

    Surgery

    Lactulose is relatively contraindicated prior to surgery, especially if the procedure requires electrocautery during proctoscopy or colonoscopy. Preoperative use of mannitol can generate a flammable gas (i.e., hydrogen gas), which can explode in the presence of a spark. Although this phenomenon has not been reported with lactulose, caution should be observed.

    Diabetes mellitus

    Although very little lactulose is absorbed systemically, it should be used with caution in diabetes mellitus patients due to its sugar content (galactose < 1.6 g/15 mL; lactose < 1.2 g/15 mL).

    Geriatric

    Geriatric or debilitated patients who receive lactulose chronically for more than 6 months should have serum electrolytes (potassium, chloride, carbon dioxide) measured periodically. The federal Omnibus Budget Reconciliation Act (OBRA) regulates the use of medications in residents of long-term care facilities. The OBRA guidelines caution that laxatives, such as lactulose, may cause flatulence, bloating, and abdominal pain.

    Pregnancy

    Lactulose is classified as an FDA pregnancy risk category B drug. No evidence of teratogenicity has been found in animal reproduction studies; the drug is virtually unabsorbed systemically. The use of lactulose may be considered during pregnancy if medically necessary. However, indiscriminate or excessive use can lead to diarrhea and dehydration, which may affect maternal nutritional and hydration status, particularly during the first trimester. Lactulose should only be used during pregnancy if the potential therapeutic benefits justify its usage, and pregnant women should seek qualified medical advice prior to use, rather than self-medicating without advice.

    Breast-feeding

    According to some manufacturers, it is not known whether lactulose is excreted in human milk. While the US manufacturer labeling advises caution, it is known that lactulose is virtually unabsorbed systemically and the expected risk to the nursing infant is very low. An occasional use to treat constipation during breast-feeding is not considered harmful and some labels do state compatibility 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.

    ADVERSE REACTIONS

    Moderate

    hypernatremia / Delayed / Incidence not known
    hypokalemia / Delayed / Incidence not known
    metabolic acidosis / Delayed / Incidence not known

    Mild

    abdominal pain / Early / 20.0-20.0
    eructation / Early / 20.0-20.0
    flatulence / Early / 20.0-20.0
    vomiting / Early / Incidence not known
    nausea / Early / Incidence not known
    diarrhea / Early / Incidence not known

    DRUG INTERACTIONS

    Acetaminophen; Hydrocodone: (Minor) Concurrent use of hydrocodone with strong laxatives that rapidly increase gastrointestinal motility, such as lactulose, may decrease hydrocodone absorption. Closely monitor patients for changing analgesic requirements or adverse events.
    Antacids: (Major) In general, other laxatives should not be used concurrently with lactulose, especially during the initial phase of therapy for portal-systemic encephalopathy, because the loose stools resulting from their use may falsely suggest that adequate lactulose dosage has been achieved. Studies suggest that oral, nonabsorbable antacids and/or laxatives like magnesium hydroxide can interfere with the decrease in colon pH necessary for lactulose's action and these alterations may make it challenging to titrate an accurate dose of lactulose during treatment of hepatic encephalopathy.
    Aspirin, ASA; Citric Acid; Sodium Bicarbonate: (Minor) Oral, nonabsorbable antacids may interfere with the decrease in colon pH necessary for lactulose's action.
    Atropine; Difenoxin: (Moderate) Diphenoxylate can decrease GI motility. Drugs used to treat constipation, such as laxatives, would counteract the effect of antidiarrheals. In general, it would be illogical to concurrently administer these drugs at the same time. If an antidiarrheal medication is needed, it would be wise to temporarily discontinue use of agents with laxative effects.
    Atropine; Diphenoxylate: (Moderate) Diphenoxylate can decrease GI motility. Drugs used to treat constipation, such as laxatives, would counteract the effect of antidiarrheals. In general, it would be illogical to concurrently administer these drugs at the same time. If an antidiarrheal medication is needed, it would be wise to temporarily discontinue use of agents with laxative effects.
    Bisacodyl: (Major) In general, other laxatives, such as bisacodyl, should not be used concurrently with lactulose, especially during the initial phase of therapy for portal-systemic encephalopathy, because the loose stools resulting from their use may falsely suggest that adequate lactulose dosage has been achieved.
    Brompheniramine; Guaifenesin; Hydrocodone: (Minor) Concurrent use of hydrocodone with strong laxatives that rapidly increase gastrointestinal motility, such as lactulose, may decrease hydrocodone absorption. Closely monitor patients for changing analgesic requirements or adverse events.
    Brompheniramine; Hydrocodone; Pseudoephedrine: (Minor) Concurrent use of hydrocodone with strong laxatives that rapidly increase gastrointestinal motility, such as lactulose, may decrease hydrocodone absorption. Closely monitor patients for changing analgesic requirements or adverse events.
    Bumetanide: (Moderate) Loop diuretics may increase the risk of hypokalemia especially in patients receiving prolonged therapy with laxatives. Monitor serum potassium levels to determine the need for potassium supplementation and/or alteration in drug therapy.
    Calcium Carbonate: (Minor) Oral, nonabsorbable antacids may interfere with the decrease in colon pH necessary for lactulose's action.
    Calcium Carbonate; Magnesium Hydroxide: (Minor) Oral, nonabsorbable antacids may interfere with the decrease in colon pH necessary for lactulose's action.
    Calcium Carbonate; Risedronate: (Minor) Oral, nonabsorbable antacids may interfere with the decrease in colon pH necessary for lactulose's action.
    Calcium Carbonate; Simethicone: (Minor) Oral, nonabsorbable antacids may interfere with the decrease in colon pH necessary for lactulose's action.
    Calcium Phosphate, Supersaturated: (Major) In general, other laxatives, such as sodium phosphate, dibasic, sodium phosphate, monobasic, should not be used concurrently with lactulose, especially during the initial phase of therapy for portal-systemic encephalopathy, because the loose stools resulting from their use may falsely suggest that adequate lactulose dosage has been achieved.
    Carbinoxamine; Hydrocodone; Phenylephrine: (Minor) Concurrent use of hydrocodone with strong laxatives that rapidly increase gastrointestinal motility, such as lactulose, may decrease hydrocodone absorption. Closely monitor patients for changing analgesic requirements or adverse events.
    Carbinoxamine; Hydrocodone; Pseudoephedrine: (Minor) Concurrent use of hydrocodone with strong laxatives that rapidly increase gastrointestinal motility, such as lactulose, may decrease hydrocodone absorption. Closely monitor patients for changing analgesic requirements or adverse events.
    Castor Oil: (Major) In general, other laxatives, such as castor oil, should not be used concurrently with lactulose, especially during the initial phase of therapy for portal-systemic encephalopathy, because the loose stools resulting from their use may falsely suggest that adequate lactulose dosage has been achieved.
    Chlorpheniramine; Guaifenesin; Hydrocodone; Pseudoephedrine: (Minor) Concurrent use of hydrocodone with strong laxatives that rapidly increase gastrointestinal motility, such as lactulose, may decrease hydrocodone absorption. Closely monitor patients for changing analgesic requirements or adverse events.
    Chlorpheniramine; Hydrocodone: (Minor) Concurrent use of hydrocodone with strong laxatives that rapidly increase gastrointestinal motility, such as lactulose, may decrease hydrocodone absorption. Closely monitor patients for changing analgesic requirements or adverse events.
    Chlorpheniramine; Hydrocodone; Phenylephrine: (Minor) Concurrent use of hydrocodone with strong laxatives that rapidly increase gastrointestinal motility, such as lactulose, may decrease hydrocodone absorption. Closely monitor patients for changing analgesic requirements or adverse events.
    Chlorpheniramine; Hydrocodone; Pseudoephedrine: (Minor) Concurrent use of hydrocodone with strong laxatives that rapidly increase gastrointestinal motility, such as lactulose, may decrease hydrocodone absorption. Closely monitor patients for changing analgesic requirements or adverse events.
    Dichlorphenamide: (Moderate) Use dichlorphenamide and lactulose together with caution. Metabolic acidosis is associated with the use of dichlorphenamide and could occur with lactulose in the presence of persistent and severe diarrhea. Concurrent use may increase the severity of metabolic acidosis. Measure sodium bicarbonate concentrations at baseline and periodically during dichlorphenamide treatment. If metabolic acidosis occurs or persists, consider reducing the dose or discontinuing dichlorphenamide therapy.
    Diphenhydramine; Hydrocodone; Phenylephrine: (Minor) Concurrent use of hydrocodone with strong laxatives that rapidly increase gastrointestinal motility, such as lactulose, may decrease hydrocodone absorption. Closely monitor patients for changing analgesic requirements or adverse events.
    Ethacrynic Acid: (Moderate) Loop diuretics may increase the risk of hypokalemia especially in patients receiving prolonged therapy with laxatives. Monitor serum potassium levels to determine the need for potassium supplementation and/or alteration in drug therapy.
    Furosemide: (Moderate) Loop diuretics may increase the risk of hypokalemia especially in patients receiving prolonged therapy with laxatives. Monitor serum potassium levels to determine the need for potassium supplementation and/or alteration in drug therapy.
    Guaifenesin; Hydrocodone: (Minor) Concurrent use of hydrocodone with strong laxatives that rapidly increase gastrointestinal motility, such as lactulose, may decrease hydrocodone absorption. Closely monitor patients for changing analgesic requirements or adverse events.
    Guaifenesin; Hydrocodone; Pseudoephedrine: (Minor) Concurrent use of hydrocodone with strong laxatives that rapidly increase gastrointestinal motility, such as lactulose, may decrease hydrocodone absorption. Closely monitor patients for changing analgesic requirements or adverse events.
    Homatropine; Hydrocodone: (Minor) Concurrent use of hydrocodone with strong laxatives that rapidly increase gastrointestinal motility, such as lactulose, may decrease hydrocodone absorption. Closely monitor patients for changing analgesic requirements or adverse events.
    Hydrocodone: (Minor) Concurrent use of hydrocodone with strong laxatives that rapidly increase gastrointestinal motility, such as lactulose, may decrease hydrocodone absorption. Closely monitor patients for changing analgesic requirements or adverse events.
    Hydrocodone; Ibuprofen: (Minor) Concurrent use of hydrocodone with strong laxatives that rapidly increase gastrointestinal motility, such as lactulose, may decrease hydrocodone absorption. Closely monitor patients for changing analgesic requirements or adverse events.
    Hydrocodone; Phenylephrine: (Minor) Concurrent use of hydrocodone with strong laxatives that rapidly increase gastrointestinal motility, such as lactulose, may decrease hydrocodone absorption. Closely monitor patients for changing analgesic requirements or adverse events.
    Hydrocodone; Potassium Guaiacolsulfonate: (Minor) Concurrent use of hydrocodone with strong laxatives that rapidly increase gastrointestinal motility, such as lactulose, may decrease hydrocodone absorption. Closely monitor patients for changing analgesic requirements or adverse events.
    Hydrocodone; Potassium Guaiacolsulfonate; Pseudoephedrine: (Minor) Concurrent use of hydrocodone with strong laxatives that rapidly increase gastrointestinal motility, such as lactulose, may decrease hydrocodone absorption. Closely monitor patients for changing analgesic requirements or adverse events.
    Hydrocodone; Pseudoephedrine: (Minor) Concurrent use of hydrocodone with strong laxatives that rapidly increase gastrointestinal motility, such as lactulose, may decrease hydrocodone absorption. Closely monitor patients for changing analgesic requirements or adverse events.
    Lanthanum Carbonate: (Minor) The manufacturer recommends that oral compounds known to interact with antacids, such as lactulose, should not be taken within 2 hours of dosing with lanthanum carbonate.
    Loop diuretics: (Moderate) Loop diuretics may increase the risk of hypokalemia especially in patients receiving prolonged therapy with laxatives. Monitor serum potassium levels to determine the need for potassium supplementation and/or alteration in drug therapy.
    Lubiprostone: (Major) In general, other laxatives, such as lubiprostone, should not be used concurrently with lactulose, especially during the initial phase of therapy for portal-systemic encephalopathy, because the loose stools resulting from their use may falsely suggest that adequate lactulose dosage has been achieved.
    Magnesium Citrate: (Major) In general, other laxatives, such as magnesium citrate, should not be used concurrently with lactulose, especially during the initial phase of therapy for portal-systemic encephalopathy, because the loose stools resulting from their use may falsely suggest that adequate lactulose dosage has been achieved.
    Magnesium Salts: (Major) In general, other laxatives, such as magnesium sulfate, should not be used concurrently with lactulose, especially during the initial phase of therapy for portal-systemic encephalopathy, because the loose stools resulting from their use may falsely suggest that adequate lactulose dosage has been achieved.
    Methylcellulose: (Major) In general, other laxatives, such as methylcellulose, should not be used concurrently with lactulose, especially during the initial phase of therapy for portal-systemic encephalopathy, because the loose stools resulting from their use may falsely suggest that adequate lactulose dosage has been achieved.
    Mineral Oil: (Major) In general, other laxatives, such as mineral oil, should not be used concurrently with lactulose, especially during the initial phase of therapy for portal-systemic encephalopathy, because the loose stools resulting from their use may falsely suggest that adequate lactulose dosage has been achieved.
    Neomycin: (Moderate) Antibiotics that reduce colonic flora theoretically interfere with the biological conversion of lactulose to its active, acidic products. Since neomycin is also used in the treatment of hepatic encephalopathy, concomitant use may interfere with the effectiveness of lactulose. Patients taking both drugs concurrently should be monitored for the possibility of a decreased response to lactulose.
    Omeprazole; Sodium Bicarbonate: (Minor) Oral, nonabsorbable antacids may interfere with the decrease in colon pH necessary for lactulose's action.
    Polycarbophil: (Major) In general, other laxatives, such as calcium polycarbophil, should not be used concurrently with lactulose, especially during the initial phase of therapy for portal-systemic encephalopathy, because the loose stools resulting from their use may falsely suggest that adequate lactulose dosage has been achieved.
    Polyethylene Glycol: (Major) In general, other laxatives, such as polyethylene glycol 3350, should not be used concurrently with lactulose, especially during the initial phase of therapy for portal-systemic encephalopathy, because the loose stools resulting from their use may falsely suggest that adequate lactulose dosage has been achieved.
    Polyethylene Glycol; Electrolytes: (Major) In general, other laxatives, such as polyethylene glycol 3350, should not be used concurrently with lactulose, especially during the initial phase of therapy for portal-systemic encephalopathy, because the loose stools resulting from their use may falsely suggest that adequate lactulose dosage has been achieved.
    Polyethylene Glycol; Electrolytes; Ascorbic Acid: (Major) In general, other laxatives, such as polyethylene glycol 3350, should not be used concurrently with lactulose, especially during the initial phase of therapy for portal-systemic encephalopathy, because the loose stools resulting from their use may falsely suggest that adequate lactulose dosage has been achieved.
    Polyethylene Glycol; Electrolytes; Bisacodyl: (Major) In general, other laxatives, such as bisacodyl, should not be used concurrently with lactulose, especially during the initial phase of therapy for portal-systemic encephalopathy, because the loose stools resulting from their use may falsely suggest that adequate lactulose dosage has been achieved. (Major) In general, other laxatives, such as polyethylene glycol 3350, should not be used concurrently with lactulose, especially during the initial phase of therapy for portal-systemic encephalopathy, because the loose stools resulting from their use may falsely suggest that adequate lactulose dosage has been achieved.
    Psyllium: (Major) In general, other laxatives, such as psyllium, should not be used concurrently with lactulose, especially during the initial phase of therapy for portal-systemic encephalopathy. Although psyllium is a bulk-forming laxative, it can cause loose stools which may falsely suggest that adequate lactulose dosage has been achieved.
    Senna: (Major) In general, other laxatives, such as senna, should not be used concurrently with lactulose, especially during the initial phase of therapy for portal-systemic encephalopathy, because the loose stools resulting from their use may falsely suggest that adequate lactulose dosage has been achieved. (Major) In general, other laxatives, such as sennosides, should not be used concurrently with lactulose, especially during the initial phase of therapy for portal-systemic encephalopathy, because the loose stools resulting from their use may falsely suggest that adequate lactulose dosage has been achieved.
    Sodium Bicarbonate: (Minor) Oral, nonabsorbable antacids may interfere with the decrease in colon pH necessary for lactulose's action.
    Sodium Phosphate Monobasic Monohydrate; Sodium Phosphate Dibasic Anhydrous: (Major) In general, other laxatives, such as sodium phosphate, dibasic, sodium phosphate, monobasic, should not be used concurrently with lactulose, especially during the initial phase of therapy for portal-systemic encephalopathy, because the loose stools resulting from their use may falsely suggest that adequate lactulose dosage has been achieved.
    Torsemide: (Moderate) Loop diuretics may increase the risk of hypokalemia especially in patients receiving prolonged therapy with laxatives. Monitor serum potassium levels to determine the need for potassium supplementation and/or alteration in drug therapy.

    PREGNANCY AND LACTATION

    Pregnancy

    Lactulose is classified as an FDA pregnancy risk category B drug. No evidence of teratogenicity has been found in animal reproduction studies; the drug is virtually unabsorbed systemically. The use of lactulose may be considered during pregnancy if medically necessary. However, indiscriminate or excessive use can lead to diarrhea and dehydration, which may affect maternal nutritional and hydration status, particularly during the first trimester. Lactulose should only be used during pregnancy if the potential therapeutic benefits justify its usage, and pregnant women should seek qualified medical advice prior to use, rather than self-medicating without advice.

    According to some manufacturers, it is not known whether lactulose is excreted in human milk. While the US manufacturer labeling advises caution, it is known that lactulose is virtually unabsorbed systemically and the expected risk to the nursing infant is very low. An occasional use to treat constipation during breast-feeding is not considered harmful and some labels do state compatibility 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

    Lactulose is a synthetic product that cannot be hydrolyzed by any gastrointestinal enzyme. Therefore, oral doses of the drug reach the colon virtually unchanged. In the colon, normal bacterial flora degrade lactulose into lactic acid and small amounts of formic and acetic acid. This acidic environment ionizes ammonia in the colon to the ammonium ion. Once ionized, ammonia, now in the form of the ammonium ion, cannot diffuse across the colon membrane and is ultimately excreted in the stool. This process reduces blood ammonia levels by 25—50%, which usually has a beneficial effect on the patient's mental status. Lactulose also can inhibit nonbacterial, glutamine-dependent ammonia production in the intestinal wall.
     
    Intracolonic breakdown of lactulose also increases osmotic pressure, causing fluid accumulation that softens the stool and distends the colon, enhancing peristalsis. Evidence suggests that reduction in luminal pH may enhance motility and secretion. Effects of lactulose occur only in the colon and are therefore motility-dependent. A normal bowel movement may not occur for 1—2 days.

    PHARMACOKINETICS

    Lactulose is administered orally or rectally. At most, 3% is absorbed into the blood and excreted unchanged in the urine within 24 hours.

    Oral Route

    Oral doses of lactulose are poorly absorbed and excreted mainly in the feces as metabolites.