A synthetic disaccharide osmotic laxative used for the treatment of chronic constipation and, at higher doses, for the treatment of hepatic encephalopathy. Lactulose is not digested by human enzymes and acts in the colon through bacterial fermentation to produce an osmotic and acidifying effect.
Evidence rating strong. Most-documented uses: relief of chronic constipation, treatment of hepatic encephalopathy, reduction of blood ammonia levels. 10 sources indexed (1992–2025), with 2 interaction records on file.
The science
How it works, mechanistically.
Core mechanism
Passes undigested to the colon, where resident bacteria metabolize it to low-molecular-weight organic acids (lactic acid, acetic acid, formic acid). This acidification increases osmotic pressure, drawing water into the colonic lumen and softening stool. In hepatic encephalopathy, the acidic environment converts ammonia (NH3) to ammonium (NH4+), which cannot be reabsorbed, thereby reducing serum ammonia levels.
Class
Osmotic Laxative
Dosing
Dosing & protocol.
Common range
Constipation: 15–30 mL (10–20 g) daily; Hepatic encephalopathy: 30–45 mL three to four times daily (as prescribed by your physician)
Recommended form
Oral solution or powder for reconstitution
May be mixed with fruit juice, water, or milk to improve palatability; can be taken with or without food
Safety
Full safety detail.
Side effects
Bloating and flatulence
Abdominal cramping
Nausea
Diarrhea (dose-dependent)
Electrolyte imbalances with excessive use
Sweet taste may be unpalatable
Contraindications
Galactosemia (contains galactose and lactose)
Patients requiring a low-galactose diet
Known bowel obstruction
Caution in diabetes (contains small amounts of free sugars)
Lactulose can cause diarrhea when the dose is too high, and excessive laxative effect can lower potassium. A published elderly-patient case involving high-dose lactulose and sorbitol reported severe hyponatremia with hypokalemia, hypocalcemia, and rhabdomyolysis. Potassium can be clinically useful when hypokalemia is confirmed, but self-treatment can overshoot in kidney disease or with potassium-raising drugs.
Recommendation: If lactulose is prescribed for hepatic encephalopathy, titrate only to the stool target your clinician gave you rather than causing continuous diarrhea. Ask about electrolyte checks if stools become very frequent, watery, or prolonged, or if you develop weakness, cramps, palpitations, dizziness, or confusion. Take potassium supplements only when your potassium level or prescriber supports it.
Lactulose and probiotics both target gut-derived ammonia pathways in hepatic encephalopathy. Trials and meta-analyses show probiotics can improve minimal hepatic encephalopathy and are often compared with lactulose; evidence for routine add-on use is less mature than evidence for lactulose itself. The combination may be reasonable in selected patients but should not replace prescribed lactulose titration.
Recommendation: Do not stop lactulose when starting a probiotic for hepatic encephalopathy. Keep lactulose titrated to the stool target your clinician gave you, commonly 2 to 3 soft stools daily. Seek urgent care for confusion, sleep-wake reversal, dehydration, severe diarrhea, or inability to take lactulose.
Moon AM, Kim HP, Jiang Y et al.. Systematic Review and Meta-Analysis on the Effects of Lactulose and Rifaximin on Patient-Reported Outcomes in Hepatic Encephalopathy. The American journal of gastroenterology. 2023
Zucker DM, Redulla R. Lactulose Management of Minimal Hepatic Encephalopathy: A Systematic Review. Gastroenterology nursing : the official journal of the Society of Gastroenterology Nurses and Associates. 2019
Luo M, Li L, Lu CZ et al.. Clinical efficacy and safety of lactulose for minimal hepatic encephalopathy: a meta-analysis. European journal of gastroenterology & hepatology. 2011
Blanc P, Daures JP, Rouillon JM et al.. Lactitol or lactulose in the treatment of chronic hepatic encephalopathy: results of a meta-analysis. Hepatology (Baltimore, Md.). 1992
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