5-Fluorouracil in Veterinary Oncology: Critical Feline Toxicity Warning

5-fluorouracil (5-FU) is covered as fluoropyrimidine antimetabolites; hazardous antineoplastic medicine, with species, evidence, regulatory, safety, interaction and monitoring boundaries for a veterinarian-led decision.

Fluoropyrimidine antimetabolites; hazardous antineoplastic medicine. Fluorouracil.

Fluorouracil.

5-Fluorouracil is a pyrimidine analogue that interferes with thymidylate synthase, DNA synthesis and RNA function.

Merck lists selected carcinomas, particularly gastrointestinal and cutaneous settings. The feline species boundary is central: this is not a drug to extrapolate from dogs to cats.

Merck lists selected carcinomas, particularly gastrointestinal and cutaneous settings. The feline species boundary is central: this is not a drug to extrapolate from dogs to cats.

Veterinary-clinic intravenous use in selected dogs; topical products create a serious household exposure hazard

Veterinary use is older, selective and extra-label. Modern treatment alternatives and formulation risks must be considered for the exact tumour. No general survival or tumour-shrinkage claim is supported.

Systemic exposure can cause nausea, vomiting, marrow suppression, gastrointestinal ulceration, neurotoxicity and liver injury. Merck states that it cannot be administered to cats because of fatal neurotoxicity. Accidental exposure to human topical cream can also be an emergency for pets.

Other marrow-suppressive or neurotoxic agents may increase risk. A household containing cats requires an explicit plan to prevent access to any topical or systemic fluorouracil product.

No FDA-approved canine or feline oncology indication was established for this medicine in this audit. In the United States, veterinary oncology use of an approved human or animal formulation is extra-label and must occur under a veterinarian within AMDUCA conditions and a valid VCPR.

Multi-region specialist use; product availability, extra-label prescribing law and pharmacy handling vary by jurisdiction.

Only a veterinary team should decide whether a canine indication justifies use. Clinic monitoring includes secure preparation and administration, blood-count and organ review, and rapid assessment of gastrointestinal or neurologic signs. Any suspected feline exposure warrants immediate emergency contact.