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Severity scale:

5-FU, also known as 5-Fluorouracil or just as Fluorouracil, is commonly used in humans to treat superficial basal cell carcinoma or actinic keratosis, and has even been used in dogs for treatment for certain cancers.  However, while the IV form at the proper dosage can be safe, eating or licking the cream that has been prescribed for you can cause some very serious side effects, ranging from stomach upset with vomiting and diarrhea to seizures and death.  Signs of toxicity have been known to appear anywhere from 30 minutes to a few hours after exposure. Any suspected ingestion of a topical chemotherapy agent warrants an immediate trip to the emergency room!

Other names: 
Ointment or topical solution used to treat skin cancer. 5-FU, Efudex, Carac, Adrucil, Fluoroplex, Tolak, topical chemotherapy
Toxic to: 
Dogs and Cats
Depression, hypersalivation, vomiting, bloody diarrhea, ataxia, tremors, hyperesthesia, hyperexcitability, seizures, pulmonary edema, respiratory failure, cadiac arrhythmias, and cardiac failure. Death may occur within 6-16 hours.
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