Lactulose
4-O-β-D-galactopyranosyl-D-fructose
Also known as: Cephulac · Constulose · Enulose · Kristalose · Generiac · Duphalac · Lactugal · Laevolac · Lactulosum
Reviewed by the VetSheet Veterinary TeamUpdated Apr 5, 2026Evidence-based veterinary reference
Audited v13 dosing evidence
Structured calculator evidenceConstipation (extra-label)
Option 1
Verified clinician required
- q6-8h
Option 2
Verified clinician required
- q8-12h
NA
Governing clinical context (2)
- NOTE: If using the crystals for oral solution: One gram of the crystals is equivalent to 1.5 mL of the liquid.
- Lactulose syrup should be stored protected from light in tight containers, preferably at room temperature (20°C-25°C [68°F-77°F]); avoid freezing. Darkening of the solution may occur, but this does not affect drug potency. Exposure to heat or light may cause extreme darkening or cloudiness; these solutions should be discarded. Lactulose solutions may precipitate; crystals dissolve with gentle reheating.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
Lactulose is a synthetic disaccharide derivative of lactose, composed of galactose and fructose. In veterinary medicine, it serves two primary, highly effective roles:
- Hepatic Encephalopathy (HE) Management: It is a cornerstone therapy for reducing blood ammonia levels in patients with compromised liver function (e.g., portosystemic shunts, severe cirrhosis). By trapping ammonia in the colon, it prevents neurotoxic accumulation in the bloodstream.
- Osmotic Laxative: It is widely used to manage chronic constipation and obstipation, particularly in cats suffering from megacolon.
Clinical Pearl: Because cats often despise the sticky, sweet syrup form of lactulose (which can lead to hypersalivation and resistance), the crystal formulation (Kristalose) mixed into wet food is often much better tolerated and improves long-term compliance.
Mechanism of action
Lactulose exerts its effects locally in the gastrointestinal tract through two distinct mechanisms:
- Ammonia Reduction (Ion Trapping): Lactulose is not hydrolyzed by mammalian gut enzymes. It reaches the colon intact, where it is fermented by resident bacteria into low-molecular-weight organic acids (lactic, formic, and acetic acids). This fermentation drops the colonic pH. The acidic environment causes diffusible, neurotoxic ammonia (NH3) to migrate from the bloodstream into the colon, where it is protonated into the non-diffusible ammonium ion (NH4+). The ammonium is then trapped in the lumen and excreted in the feces.
- Osmotic Laxative Effect: The unabsorbed disaccharide and its organic acid metabolites exert a strong osmotic pressure, drawing water into the bowel lumen. This increases fecal bulk and moisture, stimulating colonic peristalsis and facilitating bowel movements.
Safety & warnings
Contraindications
- Patients requiring a low-galactose diet
- Use with caution in diabetic patients (contains free lactose and galactose)
- Use with caution in patients with preexisting fluid and electrolyte imbalances
- Severely encephalopathic animals at risk of inhalation/aspiration
- Gastrointestinal obstruction
- Animals at risk of gastrointestinal perforation
Adverse effects
- Flatulence
- Gastric distention
- Abdominal cramping
- Diarrhea (dose-dependent)
- Dehydration (secondary to excessive diarrhea)
- Hypersalivation/aversion in cats (due to taste/texture)
- Diarrhoea
- Dehydration (with excessive dosing)
Precautions
Diabetic Patients: Lactulose syrup contains some free lactose and galactose, which may alter insulin requirements. Electrolyte Imbalances: Lactulose-induced diarrhea can exacerbate preexisting fluid and electrolyte imbalances. Monitor hydration and serum electrolytes during long-term therapy. Feline Administration: The syrup is notoriously difficult to administer to cats due to taste aversion; consider using lactulose crystals mixed with food.
Drug interactions
Non-adsorbable antacids may reduce the colonic acidification effects and overall efficacy of lactulose.
Should not be used concurrently; loose stools may be falsely attributed to lactulose, resulting in inadequate dosing for hepatic encephalopathy.
Theoretically, oral antibiotics could eliminate the colonic bacteria required to metabolize lactulose, reducing its efficacy. However, clinical synergy is often observed when treating hepatic encephalopathy. Enhanced monitoring is recommended.
Synergy may occur when used together for hepatic encephalopathy.
Lactulose syrup contains free lactose and galactose, which may alter insulin requirements in diabetic patients.
Can be inactivated because lactulose decreases the pH of the colon.
Monitoring
- Clinical efficacy: Aim for 2-3 soft, semi-formed stools per day when used for hepatic encephalopathy or constipation.
- Serum electrolytes (especially during long-term use or in patients with preexisting fluid/electrolyte abnormalities).
- Faecal consistency (aim for 2-3 soft, formed stools per day)
- Blood ammonia levels (in hepatic encephalopathy)
- Hydration status and electrolytes
- Blood glucose levels (in diabetic patients)
Pharmacokinetics
Absorption
In humans, less than 3% of an oral dose of lactulose is absorbed in the small intestine.
Distribution
Acts locally within the gastrointestinal tract; systemic distribution is minimal.
Metabolism
The absorbed fraction is not metabolized. The unabsorbed fraction reaching the colon is metabolized by resident bacteria into low molecular weight acids (lactic, formic, acetic) and CO2.
Elimination
The small absorbed fraction is excreted unchanged in the urine within 24 hours. The unabsorbed, metabolized fraction is excreted in the feces.
Overdose
Excessive doses of lactulose may cause flatulence, severe diarrhea, abdominal cramping, and dehydration.
Management:
- Reduce the dosage or temporarily discontinue the medication.
- Provide supportive care, including replacement of fluids and electrolytes if dehydration or imbalances occur.
Available products
Formulations
- Oral solution (syrup): 10 g/15 mL
- Crystals for oral solution: 10 g and 20 g packets
- Oral syrup: 3.1-3.7 g/5 ml
- Oral syrup: 10 g/15 ml
Human-labeled
- Lactulose Solution: 10 grams lactulose per 15 mL (Cephulac, Constulose, Enulose, Generiac)
- Lactulose Crystals for Oral Solution: 10 grams and 20 grams packets (Kristalose)
- Duphalac (syrup)
- Lactugal (syrup)
- Laevolac (syrup)
Regulatory status
Widely used in veterinary medicine.
Classified as 'P' (Pharmacy medicine) in the UK for human formulations often used off-label in animals.
No regulatory data for: 🇺🇸 US · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
Store lactulose syrup in tight containers, preferably at room temperature; avoid freezing. If exposed to heat or light, darkening or cloudiness of the solution may occur, but this does not affect drug potency.
Client information
What to Expect
- The Goal: The ideal outcome of lactulose therapy is 2 to 3 soft, semi-formed stools per day (think of the consistency of soft-serve ice cream).
- Adjustments: Your veterinarian will likely have you adjust the dose slightly up or down based on your pet's stool consistency.
Important Warnings
- Watch for Diarrhea: If your pet develops watery diarrhea, stop the medication and contact your veterinarian immediately, as this can lead to dangerous dehydration.
- Liver Disease: If your pet is taking this for liver disease (hepatic encephalopathy) and their neurological signs worsen (e.g., stumbling, staring into space, head pressing) or they are having fewer than 2 soft stools a day, contact your vet right away.
Tips for Cats
- Cats often hate the sticky, sweet taste of the liquid syrup and may drool excessively or run away.
- If you are struggling to give the liquid, ask your veterinarian about lactulose crystals (Kristalose), which can be easily mixed into canned food and are usually much better accepted.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.