5-fluorouracil and Pets

5-fluorouracil (5-FU) is an anti-cancer medication commonly prescribed by dermatologists to treat basal cell carcinoma and actinic keratosis on the skin of humans. Unfortunately, this medicine is highly toxic to animals. Pets develop gastrointestinal and neurologic symptoms. Gastrointestinal signs include vomiting, bloody diarrhea, intense abdominal pain and sometimes, sloughing of the intestinal lining. The neurologic signs include ataxia which means the pet walks like a drunk and seizures. I was part of a team of veterinarians who treated a dog who developed seizures after ingesting 5-FU. None of the normal medications for seizures, diazepam or phenobarbital, stopped the seizures. We had to anesthetize the dog to stop them.

Once 5-FU is ingested, it is absorbed quickly making barium or inducing vomiting, useless. Treatment is supportive which means we treat the clinical signs and support body function with fluids until the pet can fight it off. Unfortunately, the dog I mentioned above did not make it – which is not uncommon. If a pet can survive the initial phase of clinical signs, they often die from secondary effects on the bone marrow.

To protect your pet from 5-FU and other hazardous medications, please keep them in a secure location. Although it is tempting to leave the 5-FU cream on the counter, put it in your medicine cabinet. Don’t let your pet fall victim to this common toxin.

5-FU is sold under the following trade names: Efudex, Carac, Adrucil and Fluoroplex.


-Lee, Justine. ‘Top Ten Small Animal Toxins: Recognition, Diagnosis, Treatment’, ACVIM2010 Proceedings, VIN.com library.

Published by kristennelsondvm

Dr. Kristen Nelson grew up on a farm in Watertown, Minn., where she developed a deep love for animals of all kinds. She received a Doctor of Veterinary Medicine degree from the University of Minnesota, College of Veterinary Medicine. Kris then completed a small-animal internship at the prestigious Animal Medical Center in New York City. In addition to writing and speaking, she cares for small and exotic animals in Scottsdale, Az. Dr. Nelson is widely quoted in the media. Her credits include Ladies’ Home Journal, USA TODAY, the Los Angeles Times and numerous radio and television interviews. Dr. Nelson has written two books, Coated With Fur: A Vet’s Life and Coated With Fur: A Blind Cat’s Love. Kris and her husband Steve share their home with rescued cats, birds and a dog.