Lactulose: veterinary dose and monograph

At a glance

Drug class

Gastrointestinal

Also known as

Duphalac, Looz, Cephulac, Lactulose solution

Species with doses

Dogs, Cats, Horses, Birds and Reptiles

Routes

PO, Rectal

Last reviewed

1 October 2026

Indications

• Hepatic encephalopathy — reduce/eliminate gut-derived ammonia (oral and as a retention enema in crises)
• Constipation / obstipation (osmotic laxative)
• Feline megacolon management (stool softener, with prokinetics/diet)
• Stool softening where straining must be avoided

Lactulose dose by species

Doses are per administration unless stated. mg/kg doses scale with body weight; per-animal, per-quarter and infusion-rate doses are shown as the reference writes them.

Dogs

• Hepatic encephalopathy: Papich 5e: 0.5 mL/kg q8h PO. Plumb’s p.707: 0.5 mL/kg two to three times daily (Favier 2009); 1–10 mL three times daily (Twedt); reduce if diarrhoea develops.
• Constipation (laxative): Papich 5e and Plumb’s p.707 (Kirk 1986): 1 mL/4.5 kg q8h initially, then adjust.
• HE crisis — enema / stomach tube: Plumb’s p.707 (Tams 2000): in hepatic encephalopathy crisis, 20–60 mL via stomach tube every 4–6 h, or an intermittent enema diluted with water to a total of 200–300 mL.

Cats

• Constipation: Papich 5e: 1 mL/4.5 kg q8h PO. Plumb’s p.707: 0.5 mL/kg PO two to three times daily (Washabau 2007; Carr 2009); enema 5–10 mL per cat.
• Hepatic encephalopathy: Papich 5e: 2.5–5 mL per cat q8h PO. Plumb’s p.707: 0.25–1 mL per cat, individualised until semi-formed stools (Center 1986).

Horses

• Hepatic encephalopathy / laxative: Papich 5e: horses and cattle 0.25–0.5 mL/kg/day PO.

Birds

• Hepatic encephalopathy / appetite: Plumb’s p.707 (Clubb 1986): reduce if diarrhoea develops; may be used for weeks.

Reptiles

• Laxative (green iguana): Plumb’s p.707 (Wilson 2002): green iguana 0.3 mL/kg PO q12h.

Contraindications

GI obstruction. Galactosaemia (rare). Use cautiously where significant fluid/electrolyte loss is dangerous. Diabetic patients — small absorbable sugar load (usually negligible). Known hypersensitivity.

Species-specific warnings

Cats: Its very sweet taste can reduce acceptance in cats — dilute or disguise; titrate to soft (not liquid) stool.

Adverse effects

• Diarrhoea, flatulence and abdominal cramping (dose-related)
• Dehydration and hypernatraemia/electrolyte loss with overuse
• Reduced palatability (very sweet) in some cats

Drug interactions

• Oral antibiotics (e.g. neomycin): theoretical reduction of fermenting flora, but the combination is used together in HE and is generally complementary
• Other laxatives: additive diarrhoea
• Antacids: may blunt the desired colonic acidification (minor)

Pregnancy and lactation

Considered safe in pregnancy and lactation (not systemically absorbed).

Monitoring

• Stool number/consistency — titrate to 2–3 soft stools/day (not watery diarrhoea)
• Neurological status / blood ammonia in hepatic encephalopathy
• Hydration and electrolytes if high doses or diarrhoea
• Body weight / appetite

Toxicity and overdose

Very wide safety margin. Overuse simply causes diarrhoea, with secondary dehydration, hypernatraemia and electrolyte disturbance if excessive. No systemic toxicity because it is not absorbed.

Management: Stop or reduce the dose; provide fluid and electrolyte support to correct diarrhoea-related losses. No specific antidote needed. Resume at a lower dose titrated to 2–3 soft stools per day.

Clinical pearls

• Dose to EFFECT (2–3 soft stools daily), not to a number — that is the whole art of lactulose use
• The backbone of hepatic-encephalopathy therapy: it both empties the colon and traps ammonia as ammonium
• In an HE crisis, a diluted lactulose RETENTION ENEMA works faster than oral dosing
• Pair with a low-(quality)-protein diet ± gut antibiotic for portosystemic-shunt/liver-failure patients
• Very safe — the worst common problem is over-shooting into watery diarrhoea and dehydration

Mechanism of action

Lactulose is not digested by mammalian enzymes; colonic bacteria ferment it to short-chain (lactic, acetic) acids. This (1) draws water into the lumen osmotically — the laxative effect — and (2) acidifies colonic contents, converting absorbable ammonia (NH3) into non-absorbable ammonium (NH4+) which is trapped and excreted, while the lowered pH and faster transit also reduce ammonia-producing flora. The net effect is a fall in blood ammonia in hepatic encephalopathy.

Formulations and products

• Oral solution/syrup ~667 mg/mL (10 g/15 mL)
• Duphalac — 10 g/15 mL solution, Oral solution/syrup (human-label)
• Looz / Livoluk — 10 g/15 mL solution, Oral solution (human-label)

Storage

Store at 15–30 °C; do not freeze. May darken with age/heat (still usable unless very dark). Keep tightly closed.

Before you use this dose

Reference doses for veterinary professionals. Confirm the dose against the product label, the patient’s condition, current guidance and local regulations before use; the attending veterinarian remains responsible for every clinical decision.

Other gastrointestinal drugs

• Cimetidine
• Cisapride
• Famotidine
• Hyoscine Butylbromide (N-Butylscopolammonium)
• Loperamide
• Metoclopramide
• Metronidazole
• Mineral Oil (Liquid Paraffin)

Frequently asked questions