It is extremely toxic to rapidly dividing cells and can cause severe gastrointestinal symptoms. Toxic doses can be as little as 5 mg/kg, and doses ≥40 mg/kg are reported to be fatal. Unfortunately, pharmacists and physicians do not always warn of the potential toxicity some medicines may pose to pets.

(CNN)The Food and Drug Administration warned pet owners Wednesday that dogs, cats and other animals may die if exposed to skin cancer creams. It may be sold under the names Carac, Efudex and Fluoroplex, all of which contain the active ingredient fluorouracil, also known as 5-FU, and it may be a danger to pets.

The FDA received reports of five dogs becoming ill and dying after consuming skin creams containing fluorouracil.
In one case, a playful dog punctured a tube of fluorouracil cream. Within two hours, the animal began vomiting, experienced seizures and died 12 hours later.
In another case, a dog ingested a tube of the cream. Though the owner rushed the dog to a veterinarian, who immediately began to treat the animal, the dog's condition worsened and after three days, the owner deemed it necessary to euthanize the pet