Lactulose

Osmotic laxative, Non-absorbable disaccharide, Hepatic encephalopathy treatment

Lactulose

Kristalose, Constulose, Generlac, Enulose

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Summary

Classes

Osmotic laxative, Non-absorbable disaccharide, Hepatic encephalopathy treatment

Common uses

Chronic constipation▲0▼, Hepatic encephalopathy (acute and chronic)▲0▼

Pharmacy

Starting dose

Constipation: 15-30 mL PO daily (titrate to 1-2 soft stools/day); hepatic encephalopathy: 20-30 g (30-45 mL) PO/PR every 1-2 hours acutely until soft stools, then BID-QID to target 2-3 soft stools/day

Preparations

10 g/15 mL solution (Constulose); 10 g, 20 g powder packets (Kristalose)

US FDA Max

Indication-specific; HE may require high-volume dosing

Pharmacology

Routes

Oral, rectal (enema, especially in acute HE)

Onset

Constipation: 24-48 hours; HE: ammonia reduction within hours of stool production

Duration

Hours

Half-life

Not meaningfully described — lactulose is not significantly absorbed

Bioavailability

<3% systemic absorption (the basis of the safety and mechanism)

Pregnancy

Generally considered safe (minimal systemic absorption).^{[citation needed]}

Legal status

Rx-only in US

Purported mechanism

Lactulose is a non-absorbable synthetic disaccharide (galactose-fructose) that passes intact to the colon where bacterial fermentation produces short-chain fatty acids, lowering luminal pH and trapping ammonia as non-absorbable ammonium ion (NH₄⁺); the same fermentation also produces an osmotic load that draws water into the lumen, softening stool.▲0▼ In hepatic encephalopathy, both mechanisms (ammonia trapping AND laxation that purges nitrogenous waste from the gut) contribute to ammonia reduction. Cathartic flatulence is the universal side effect; dehydration with overuse can paradoxically precipitate HE.