Osmotic Laxative — Constipation / Hepatic Encephalopathy
Pregnancy: Safe — lactulose is not systemically absorbed; widely used for constipation in pregnancy
Lactulose (Paediatric)
Brand names: Duphalac, Lactugal
Adult dose
Dose: Constipation: 15 mL twice daily; Hepatic encephalopathy: 30–45 mL 3 times daily
Route: Oral
Frequency: Twice to three times daily
Max: 200 mL/day (encephalopathy)
Adult reference — see paediatric dose section
Paediatric dose
Route: Oral (solution — 3.35 g/5 mL)
Frequency: Twice daily (adjust to produce 1–2 soft stools/day)
Max: Age-banded
BNFc: Under 1 year: 2.5 mL twice daily; 1–5 years: 2.5–10 mL twice daily; 5–18 years: 5–20 mL twice daily. Adjust dose every 2 days to achieve soft stools without diarrhoea. Takes 2–3 days for full effect. Not first-line for constipation in children per NICE — macrogol (polyethylene glycol) is first-line for childhood constipation; lactulose is second-line or for infants <1 year. May cause flatulence and bloating — warn parents.
Dose adjustments
Renal
No dose adjustment required
Hepatic
Used therapeutically in hepatic encephalopathy — no dose adjustment
Clinical pearls
- NICE NG90: macrogol (Movicol Paediatric Plain) is first-line for childhood constipation — superior evidence, more predictable, better tolerated; lactulose second-line or for infants <1 year where macrogol sachets less practical
- Mechanism: lactulose reaches colon undigested (no human lactulase), fermented by colonic bacteria to lactic/acetic acids — osmotic effect draws water into colon; additionally reduces stool pH and nitrogen absorption (hepatic encephalopathy mechanism)
- Hepatic encephalopathy: target 2–3 soft stools per day — traps ammonium ions in colon (pH-dependent ionisation prevents reabsorption), reduces nitrogen load to liver
- Flatulence management: start at lowest dose, increase gradually; give with meals; some families cannot tolerate due to gas — switch to macrogol
Contraindications
- Galactosaemia
- Intestinal obstruction
- Galactose or lactose intolerance
Side effects
- Flatulence (very common — from colonic fermentation)
- Abdominal bloating and cramps
- Diarrhoea (excess doses)
- Nausea
- Electrolyte disturbance (prolonged high-dose use)
Interactions
- Antacids — may reduce lactulose efficacy
- Antibiotics — neomycin/metronidazole may reduce lactulose efficacy in hepatic encephalopathy (reduce gut flora needed for fermentation)
Monitoring
- Stool frequency and consistency (Bristol Stool Scale)
- Electrolytes (prolonged high-dose use)
- Signs of dehydration
- Ammonia levels (hepatic encephalopathy indication)
Reference: BNF for Children; NICE NG90 (Constipation in Children); ESPGHAN Childhood Constipation Guidelines 2014. Verify against your local formulary and the latest BNF before prescribing.
Related
Curated clinical cross-links plus same-class fallbacks.
Calculators