Lactulose !
A sugar the gut cannot absorb: it softens stool and, in liver disease, traps ammonia in the colon to prevent and treat hepatic encephalopathy.
Updated 2026-11-01
Mechanism of action
Lactulose is a synthetic disaccharide that no human enzyme can split, so it reaches the colon unchanged. Colonic bacteria ferment it to lactic acid and smaller amounts of formic and acetic acid. The acids draw water into the bowel and soften the stool. The acid colon also turns ammonia (NH₃) into ammonium (NH₄⁺), which cannot be absorbed; ammonia moves from the blood into the colon and is flushed out with the stool, lowering blood ammonia.
- Reaches the colon intact: no human enzyme splits it
- Colonic bacteria ferment it into lactic and other organic acids
- The acid colon traps ammonia as non-absorbable ammonium (NH₄⁺)
- Osmotic water and faster transit expel the ammonium in the stool
Pharmacokinetics
- Absorption
- Very little is absorbed; it reaches the colon virtually unchanged
- Metabolism
- Broken down by colonic bacteria, not by human enzymes
- Elimination
- Mostly in stool; the small absorbed fraction leaves in urine
- Onset and duration
- For constipation, a bowel movement may take 24 to 48 hours
Indications
- Prevention and treatment of hepatic (portal-systemic) encephalopathy, including precoma and coma Hepatic encephalopathy →
- Long-term prevention of recurrent encephalopathy in cirrhosis Cirrhosis →
- Constipation Constipation and faecal impaction →
Adverse effects
Common
- Flatulence, belching and bloating
- Abdominal cramps
- Nausea and vomiting
- Diarrhoea
Contraindications
- Patients who need a low-galactose diet (the solution contains galactose)
Key interactions
- Non-absorbable antacids
- May block the drop in colonic pH and reduce the effect
- Oral anti-infectives such as neomycin
- May remove the colonic bacteria that break lactulose down; response is watched closely
- Other laxatives
- Loose stools may falsely suggest that lactulose is working in hepatic encephalopathy
What is monitored
- Number and consistency of stools (target set by the care team)
- Mental status and degree of encephalopathy
- Hydration and electrolytes (potassium, sodium), especially with long use in older or frail patients
- Blood glucose in diabetes (the solution contains galactose and lactose)
Administration and safety
- Given by mouth or, in impending coma or coma with risk of aspiration, as a retention enema through a rectal balloon catheter.
- Do not use soapsuds or other alkaline cleansing enemas with it: they cancel the acidification.
- Before colonoscopy with electrocautery, the bowel is cleansed with a non-fermentable solution because lactulose can produce hydrogen gas.
Nursing care
- Assess orientation, alertness and stool pattern before each dose in hepatic encephalopathy.
- Record every stool; report persistent diarrhoea, which signals excessive dosing.
- Watch for dehydration and check potassium and sodium results.
- Assess swallowing and aspiration risk in a drowsy patient; the rectal route may be needed.
- Teach that gas and cramps are common at the start and usually pass.
Pregnancy and breastfeeding
Very little reaches the blood and animal studies showed no harm; it is used in pregnancy when needed and is compatible with breastfeeding.
Test yourself
Think of the answer, then tap the question to see it.
How does lactulose lower blood ammonia?
Colonic bacteria turn it into acids; the acid colon converts ammonia into ammonium, which cannot be absorbed and is expelled in the stool.
Why can lactulose not be absorbed in the small intestine?
No human enzyme splits this disaccharide, so it reaches the colon unchanged.
How can lactulose be given to a patient in hepatic coma who cannot swallow safely?
As a retention enema through a rectal balloon catheter.
Why are other laxatives avoided when starting lactulose for hepatic encephalopathy?
Their loose stools can falsely suggest that the lactulose dose is adequate.
Related drugs
Related conditions
← Back to the class: Laxatives
Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.
Sources: DailyMed - LACTULOSE- lactulose solution (DailyMed (U.S. National Library of Medicine), 2022); DailyMed - LACTULOSE solution (DailyMed (U.S. National Library of Medicine), 2026); Lactulose: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2024); Lactulose: a laxative medicine to treat constipation - NHS (NHS (National Health Service, UK), 2026).
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