Lactulose
**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: 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.
Dosing by species
- Hepatic encephalopathy ยท 0.25-1 mL ยท PO ยท As needed ยท Individualize dosage until semi-formed stools are produced
- Constipation ยท 1/2 to 3/4 of a teaspoonful ยท PO ยท twice daily ยท Suggested dose
- Constipation ยท 5-10 mL per cat ยท Rectal ยท Once ยท Administered slowly as an enema with a well-lubricated 10-12 (french) rubber catheter or feeding tube
- Constipation ยท 0.5 mL/kg ยท PO ยท two to three times a day
- Chronic constipation (maintaining soft stools) ยท 0.5 mL/kg ยท PO ยท two to three times daily ยท Dosage is adjusted to obtain the stool quality desired
- Hepatic encephalopathy / Constipation ยท Dose not specified in text. Titrate to effect (reduce dose if diarrhoea develops). ยท PO ยท Not specified ยท Ongoing ยท Alternative provided in text: Lactitol (ฮฒ-galactosidosorbitol) powder at 500 mg/kg/day divided in 3 or 4 doses, adjusted to produce 2 or 3 soft stools per day.
- Hepatic encephalopathy; to stimulate appetite, improve intestinal flora ยท 0.03 mL (Cockatiel); 0.1 mL (Amazon) ยท PO ยท two to three times a day ยท May be used for weeks ยท Reduce dosage if diarrhea develops
- Laxative ยท 0.3 mL/kg ยท PO ยท q12h ยท Green Iguana
Routes of administration
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)
Drug interactions
- Oral Antacids ยท Non-adsorbable antacids may reduce the colonic acidification effects and overall efficacy of lactulose. ยท moderate
- Other Laxatives ยท Should not be used concurrently; loose stools may be falsely attributed to lactulose, resulting in inadequate dosing for hepatic encephalopathy. ยท moderate
- Neomycin (and other oral antibiotics) ยท 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.
- Oral antibiotics (e.g., neomycin) ยท Synergy may occur when used together for hepatic encephalopathy. ยท minor
- Insulin ยท Lactulose syrup contains free lactose and galactose, which may alter insulin requirements in diabetic patients. ยท moderate
- 5-aminosalicylic acid (and other pH-dependent colon-release drugs) ยท Can be inactivated because lactulose decreases the pH of the colon. ยท major
Monitoring
- Clinical efficacy: Aim for 2-3 soft, semi-formed stools per day when used for hepatic encephalopathy or constipation.
- Hydration status.
- 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)
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.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturerโs current label.