Lactulose

Lactulose is a poorly absorbed synthetic disaccharide used as an osmotic laxative and as part of hepatic encephalopathy treatment. Its colonic fermentation products increase stool water and alter the handling of intestinal ammonia. In veterinary practice, major applications include constipation in dogs and cats and hyperammonemia associated with portosystemic shunting or advanced liver disease.

Group

Osmotic laxative and ammonia-trapping agent

Mechanism of Action

Colonic bacteria ferment lactulose into organic acids that increase luminal osmotic activity and lower colonic pH. Water retention softens stool and accelerates transit. Acidification shifts diffusible ammonia toward less readily absorbed ammonium, while altered bacterial metabolism and faster transit can reduce the intestinal ammonia burden. These combined effects support both laxation and reduction of ammonia absorption.

Dose Rates

Dogs

Use / stage	Dose	Route	Schedule	Instructions
Constipation	0.1481481481 g/kg	PO	Every 8 h	Equivalent to 1 mL per 4.5 kg of the 10 g/15 mL solution; titrate to stool response.
Hepatic encephalopathy	0.3333333333 g/kg	PO	Every 8 h	Equivalent to 0.5 mL/kg of the 10 g/15 mL solution.
Hepatic encephalopathy	10–20 g/animal	PO	Every 6 h	Equivalent to 15-30 mL of 10 g/15 mL solution; titrate to 2-3 soft stools daily.
Hepatic encephalopathy	0.6666666667–6.6666666667 g/animal	PO	Every 8 h	Equivalent to 1-10 mL three times daily; titrate stool consistency.
Hepatic encephalopathy	0.3333333333 g/kg	PO	Every 8–12 h	Equivalent to 0.5 mL/kg two or three times daily.

Cats

Use / stage	Dose	Route	Schedule	Instructions
Constipation	0.1481481481 g/kg	PO	Every 8 h	Equivalent to 1 mL per 4.5 kg of the 10 g/15 mL solution; titrate to stool response.
Hepatic encephalopathy	1.6666666667–3.3333333333 g/animal	PO	Every 8 h	Equivalent to 2.5-5 mL/cat of the 10 g/15 mL solution.
Hepatic encephalopathy retention enema	Reference only	RECTAL		20-30 mL/kg of a 30% solution; this is a local-procedure volume instruction, different from the oral solution dose.
Constipation	0.3333333333 g/kg	PO	Every 8–12 h	Equivalent to 0.5 mL/kg of 10 g/15 mL solution; titrate to stool response.
Hepatic encephalopathy	0.1666666667–0.6666666667 g/animal	PO		Equivalent to 0.25-1 mL; repeat frequency must be individualized because it is not specified.
Constipation enema	Reference only	RECTAL		5-10 mL/cat, slowly through a well-lubricated 10-12 French soft catheter; use an appropriate retention plan.

Horses

Use / stage	Dose	Route	Schedule	Instructions
Laxative treatment	0.1666666667–0.3333333333 g/kg	PO	Every 24 h	Equivalent to 0.25-0.5 mL/kg/day of the 10 g/15 mL solution.

Cattle

Use / stage	Dose	Route	Schedule	Instructions
Laxative treatment	0.1666666667–0.3333333333 g/kg	PO	Every 24 h	Equivalent to 0.25-0.5 mL/kg/day of the 10 g/15 mL solution.

Reptiles

Use / stage	Dose	Route	Schedule	Instructions
Green-iguana constipation	0.2 g/kg	PO	Every 12 h	Green iguanas only; equivalent to 0.3 mL/kg of 10 g/15 mL solution.

Birds

Use / stage	Dose	Route	Schedule	Instructions
Hepatic encephalopathy or appetite adjunct	Reference only	PO		Cockatiels 0.03 mL; Amazons 0.1 mL, two or three times daily of the described lactulose liquid. Reduce if diarrhoea develops.

Indications

Treatment of constipation in dogs and cats, including selected chronic constipation or megacolon cases.
Reduction of intestinal ammonia absorption in hepatic encephalopathy associated with portosystemic shunting or severe hepatic dysfunction.
Prevention of recurrent hyperammonemic signs in selected patients with an established hepatic encephalopathy treatment plan.

Contraindications

Use cautiously with diabetes because some products contain absorbable sugars.

Interactions

Changes in intestinal pH and transit may change absorption of other oral drugs.

Adverse effects

Diarrhoea and excessive fluid or electrolyte loss

Toxicity

Reduce or stop treatment if marked diarrhoea or dehydration occurs and replace fluid and electrolytes as indicated.

Preparations

Preparation	Form	Strength	Route
Lactulose 10 g/15 mL solution	Oral Liquid	10 g / 15 mL	PO
Lactulose 10 g crystals sachet	Granules	10 g / sachet	PO
Lactulose 20 g crystals sachet	Granules	20 g / sachet	PO

Monitoring and Administration

Titrate to stool response and monitor hepatic encephalopathy clinically.

Mass equivalents below use exactly 10 g per 15 mL; do not substitute a differently concentrated product by volume.

Darkening alone may not reduce potency. Keep tightly closed and avoid freezing.

Lactulose crystals: 1 g corresponds to 1.5 mL of the 10 g/15 mL liquid. Canine crisis stomach-tube volumes of 20-60 mL every 4-6 hours require individual titration. The enema total-volume statement paired with “300-450 grams” is dimensionally inconsistent and is not used as a mixing recipe.