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Lactulose for Veterinary Use

by Barbara Forney, VMD

Basic Information

Lactulose is a synthetic non-digestible disaccharide which is used to treat hepatic encephalopathy (HE) and chronic constipation in dogs and cats. Oral lactulose is very poorly absorbed and the majority of the drug remains in the colon where it is metabolized by the resident bacterial flora into short chain fatty acids. These short chain fatty acids acidify the colon contents and draw water and ammonia (NH3) from the blood into the colon. The net effect is to soften the stool and reduce blood ammonia levels by trapping and expelling ammonium ion in the feces.

Dogs and Cats

Lactulose is used to treat HE in both dogs and cats. The most common cause of HE is the presence of a congenital portal-caval shunt. Other causes of HE include acute or chronic liver failure, and juvenile fibrosing liver disease. Lactulose may be administered orally or used in a retention enema during an emergency situation such as a seizuring animal or during a hepatic coma.

Lactulose is also used as a stool softener in the treatment of chronic mega-colon in both dogs and cats, and in the treatment of rectal-anal polyps in dogs.

Lactulose Side Effects

  • Gastro-intestinal side effects including flatulence, GI distension, and cramping are relatively common at the beginning of treatment.
  • Diarrhea, and/or dehydration may indicate the need for a reduced dose.


  • Lactulose should be used with additional caution in animals with dehydration or electrolyte abnormalities.
  • Lactulose may change the insulin requirements in some diabetic animals.
  • Lactulose should not be used with other laxatives.
  • There have not been adequate studies on the use of lactulose in pregnant or nursing animals but because it is very poorly absorbed from the GI tract, the risk is thought to be relatively low.
  • Drug Interactions

  • Oral antacids may decrease the acidification of colon contents and possibly decrease the efficacy of lactulose.
  • Neomycin and other oral antibiotics may change the colonic bacterial flora and possibly decrease the efficacy of lactulose.


Diarrhea and dehydration may occur in an overdose situation. Replacement fluids, electrolytes and other supportive measures may be indicated.

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Therapeutic Class
Synthetic disaccharide

Dogs and cats

May Be Prescribed by Vets for:
Reduction of blood ammonia levels, and osmotic laxative

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About the Author

Dr. Barbara Forney

Barbara Forney, VMD

Dr. Barbara Forney is a veterinary practitioner in Chester County, Pennsylvania. She has a master's degree in animal science from the University of Delaware and graduated from the University of Pennsylvania School of Veterinary Medicine in 1982.

She began to develop her interest in client education and medical writing in 1997. Recent publications include portions of The Pill Book Guide to Medication for Your Dog and Cat, and most recently Understanding Equine Medications published by the Bloodhorse.

Dr. Forney is an FEI veterinarian and an active member of the AAEP, AVMA, and AMWA.