Osmotic laxative / Ammonia reduction (hepatic encephalopathy)

Lactulose

Brand names: Lactulose, Duphalac

Lactulose is an osmotic laxative used for constipation and for the treatment and prevention of hepatic encephalopathy.

Dosing — being independently re-sourced

ClinCalc Pro is rebuilding its dose data from primary open sources — the manufacturer SmPC (eMC), the WHO Model Formulary and other official references — under clinician review. This drug's structured dose is not yet published here. Confirm all doses against the product SmPC and your local formulary before prescribing.

US labelling (FDA)

Reference — US labelling, may differ from UK

DOSAGE AND ADMINISTRATION Oral Adult: The usual adult, oral dosage is 2 to 3 tablespoonfuls (30 to 45 ml, containing 20 g to 30 g of lactulose) three or four times daily. The dosage may be adjusted every day or two to produce 2 or 3 soft stools daily. Hourly doses of 30 to 45 ml of Lactulose may be used to induce the rapid laxation indicated in the initial phase of the therapy of portal-systemic encephalopathy. When the laxative effect has been achieved, the dose of lactulose may then be reduced to the recommended daily dose. Improvement in the patient's condition may occur within 24 hours but may not begin before 48 hours or even later. Continuous long-term therapy is indicated to lessen …

Source: US FDA prescribing information (openFDA / DailyMed), label dated 2022-06-01. Accessed 2026-06-12. US dosing and indications can differ from UK practice — use UK sources for prescribing decisions.

Clinical monograph

How it works

A non-absorbed disaccharide that osmotically draws water into the bowel; in hepatic encephalopathy it also acidifies the colon and reduces ammonia absorption.

Prescribing in practice

For constipation it may take a day or two to work and commonly causes flatulence and cramps initially.
Avoid in galactosaemia and intestinal obstruction.
In hepatic encephalopathy the dose is titrated to achieve a target number of soft stools per day.

Monitoring

Review bowel habit; in hepatic encephalopathy titrate to the target stool frequency and monitor mental state.

Counselling the patient

It can take 1-2 days to work.
Some bloating or wind is common at first and usually settles.
Keep up your fluid intake.

Evidence & guidelines

A first-line osmotic laxative for constipation and standard therapy in hepatic encephalopathy.

Reference: EASL HE Guidelines; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. Monograph status: clinician-reviewed (2026-07-04).

Curated clinical cross-links plus same-class fallbacks.

Calculators

Pathways