Fluorouracil should not be given to pregnant or nursing dogs, due to it being a teratogen, which lead to a number of different birth defects. It should also not be given to patients suffering from bone marrow suppression, serious infections, poor nutritional states, or a known hypersensitivity to antimetabolites. Patients with blood disorders should only be given fluorouracil under certain conditions, and should be used with great caution. Use with Leucovorin might increase the chances of gastrointestinal toxicity. Also, fluorouracil should never be given to cats, as it is extremely neurotoxic.