5-Fluorouracil Toxicosis in Cats and Dogs

5-Fluorouracil (5-FU) is a cytotoxic chemotherapy agent. Common brand names include Adrucil® and Efudix®. 5-FU is formulated as a topical cream or solution and an injectable solution. The cream formulation is more commonly found as a household product. 5-FU is used in veterinary medicine for equine sarcoids and squamous cell carcinoma.  Cats and dogs are often exposed when chewing on a tube of the product, and less likely by topical application. When ingested by dogs or cats, it is likely to be fatal. If your pet has ingested 5-FU, they should be seen by a veterinarian immediately, regardless of how much they consumed. Cats are highly sensitive to 5-FU, even in very small amounts and often die acutely from central nervous system toxicity.


Common clinical signs of 5-FU exposure include vomiting +/- blood, diarrhea, depression, tremors, respiratory distress, noncontrollable seizures, and decreased or elevated heart rate. Clinical signs usually present within 30-60 minutes but can be delayed up to 6 hours. Many animals die within 24 hours of ingestion. If the patient survives, there is a risk of delayed bone marrow suppression and bleeding.


Due to the rapid onset of neurologic signs, inducing vomiting at home is only recommended if your dog is not showing any clinical signs and consumed the product less than 10 minutes ago. DO NOT attempt to induce vomiting in cats. Your pet should be brought to the nearest veterinarian as quickly as possible so immediate aggressive treatment can be started.  Treatment may include medications to treat and prevent seizures, gastrointestinal protectants, fluid therapy, and potentially blood transfusions or antibiotics.


If you or someone you know has been prescribed 5-Fluorouracil, be sure to keep it secured and out of pets reach. Do not leave it in an area accessible to your pet. Be sure to wear gloves when applying the medication and promptly dispose of the gloves in a trash that your pet cannot access, ideally behind a close door. Do not allow your pet to lick where you have applied 5-FU. If you are concerned that your pet may have ingested or come in contact with 5-FU, immediately bring them to the nearest veterinarian and contact Pet Poison Helpline.


Written by:

Anne Desalvo, Pet Poison Helpline DVM student extern, University of Minnesota DVM Candidate 2023

Sam Koch, CVT, Veterinary Information Specialists, Pet Poison Helpline