# Lactulose: veterinary dose and monograph

> Lactulose is a veterinary gastrointestinal drug. A non-absorbable synthetic disaccharide (galactose-fructose) with two roles: an osmotic LAXATIVE (constipation, megacolon adjunct) and the cornerstone medical therapy for HEPATIC ENCEPHALOPATHY, where it traps and eliminates ammonia from… Indications include 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). This monograph lists 8 reference doses for dogs, cats, horses, birds and reptiles, each with route, frequency and source.

Source: https://vetmasterji.com/drugs/lactulose · Last reviewed: 2026-10-01

## 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

*Lactulose doses for dogs*

| Indication | Dose | Route | Frequency | Duration |
| --- | --- | --- | --- | --- |
| Hepatic encephalopathy | 0.5 mL/kg PO q8–12h, titrated to 2–3 soft stools a day | PO | q8–12h | — |
| Constipation (laxative) | 1 mL per 4.5 kg PO q8h, adjusted to effect | PO | q8h | — |
| HE crisis — enema / stomach tube | 20–60 mL by stomach tube q4–6h, or as an intermittent diluted enema (total 200–300 mL) | Rectal | q4–6h as needed | — |

- 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

*Lactulose doses for cats*

| Indication | Dose | Route | Frequency | Duration |
| --- | --- | --- | --- | --- |
| Constipation | 0.5 mL/kg PO q8–12h (or 1 mL per 4.5 kg q8h), adjusted to stool quality | PO | q8–12h | — |
| Hepatic encephalopathy | 2.5–5 mL per cat PO q8h, titrated to semi-formed stools | PO | q8h | — |

- 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

*Lactulose doses for horses*

| Indication | Dose | Route | Frequency | Duration |
| --- | --- | --- | --- | --- |
| Hepatic encephalopathy / laxative | 0.25–0.5 mL/kg/day PO | PO | daily (divided) | — |

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

### Birds

*Lactulose doses for birds*

| Indication | Dose | Route | Frequency | Duration |
| --- | --- | --- | --- | --- |
| Hepatic encephalopathy / appetite | Cockatiel 0.03 mL; Amazon parrot 0.1 mL PO two to three times daily | PO | q8–12h | — |

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

### Reptiles

*Lactulose doses for reptiles*

| Indication | Dose | Route | Frequency | Duration |
| --- | --- | --- | --- | --- |
| Laxative (green iguana) | 0.3 mL/kg PO q12h | PO | q12h | — |

- 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

> **Caution:** 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

> **Caution:** 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.

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## Frequently asked questions

### What is the dose of Lactulose for dogs?

Reference doses for Lactulose in dogs: 0.5 mL/kg PO q8–12h, titrated to 2–3 soft stools a day PO q8–12h (Hepatic encephalopathy); 1 mL per 4.5 kg PO q8h, adjusted to effect PO q8h (Constipation (laxative)); 20–60 mL by stomach tube q4–6h, or as an intermittent diluted enema (total 200–300 mL) Rectal q4–6h as needed (HE crisis — enema / stomach tube). Confirm against the label and the patient before use.

### What is the dose of Lactulose for cats?

Reference doses for Lactulose in cats: 0.5 mL/kg PO q8–12h (or 1 mL per 4.5 kg q8h), adjusted to stool quality PO q8–12h (Constipation); 2.5–5 mL per cat PO q8h, titrated to semi-formed stools PO q8h (Hepatic encephalopathy). Confirm against the label and the patient before use.

### What is the dose of Lactulose for horses?

Reference doses for Lactulose in horses: 0.25–0.5 mL/kg/day PO PO daily (divided) (Hepatic encephalopathy / laxative). Confirm against the label and the patient before use.

### When should Lactulose not be used?

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

### What are the side effects of Lactulose in animals?

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

## Sources

- Plumb's Veterinary Drug Handbook
- Merck Veterinary Manual, 11th edition
- Papich, Papich Handbook of Veterinary Drugs, 5th edition

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VetMasterJi is a clinical decision-support and education platform for veterinary professionals and students. It does not provide veterinary advice, diagnosis or treatment for specific animals. All drug doses, calculators and differentials are references that must be independently verified before clinical use; the attending registered veterinarian remains solely responsible for every clinical decision.