Fluorouracil
Also known as: Adrucil, Efudix, Fluorouracil, 5-FU
> **CRITICAL WARNING FOR CAT OWNERS**: Human topical creams containing 5-FU (used for skin lesions) are **highly toxic and frequently fatal** to cats. Even a tiny lick or walking across a treated area and grooming can cause death. Keep all such medications strictly out of reach. * **Serious Side Effects**: Understand that this is a potent chemotherapy drug. It can cause severe gastrointestinal upset, bone marrow suppression (increasing the risk of infection), and neurological signs. * **Monitoring**: Strict adherence to follow-up appointments for bloodwork (CBCs) is mandatory to ensure your pet's white blood cell counts remain safe. * **Safety at Home**: Because this drug is a known teratogen (can cause birth defects) and is excreted in bodily fluids, pregnant women, nursing mothers, and children should avoid handling the medication or the pet's waste (urine/feces) for several days post-treatment. Wear gloves when cleaning up accidents.
Dosage must be set by your veterinarian for your specific pet. Never give human medication or another pet’s prescription without veterinary guidance.
What it is used for
- ALL
- All indications
- Intratumoral injection with epinephrine into certain skin neoplasms (squamous cell carcinoma, melanoma, sarcoid)
- Canine mammary carcinoma (in combination with doxorubicin and cyclophosphamide-FAC protocol), dermal squamous cell carcinoma and GI tract tumors
- Systemic carcinomas
- Basal cell and squamous cell carcinoma
How it is given
Possible side effects
- Dose-dependent myelosuppression (leukopenia, thrombocytopenia)
- Gastrointestinal toxicity (nausea, vomiting, hemorrhagic diarrhea, GI ulceration/sloughing, stomatitis)
- Neurotoxicity (ataxia, disorientation, seizures)
- Fatal neurotoxicity in cats (even with topical exposure)
- Anorexia
- Vomiting
- Stomatitis
- Diarrhoea
- Leucopenia (nadir between 7 and 14 days)
- Thrombocytopenia
- Anaemia
- Alopecia
- Hyperpigmentation
- Dermatitis
Cautions & contraindications
- CATS (Do NOT use in any form, including topical)
- Patients with known hypersensitivity to the drug
- Patients in poor nutritional states
- Patients with depressed or reduced bone marrow function
- Patients with concurrent serious infections
- Pregnancy (Known teratogen - FDA Category D)
- Cats (strictly contraindicated due to fatal neurotoxicity)
- Patients with severe bone marrow suppression
- Patients with severe hepatic or renal impairment (unless dose is adjusted)
- Pregnancy and lactation
- Fluorouracil has a very narrow therapeutic index and should only be administered by clinicians experienced with cancer chemotherapeutic agents. **Feline Toxicity**: Cats develop severe, potentially fatal neurotoxicity. It is contraindicated in cats in any form. **Reproductive Safety**: 5-FU is a known teratogen. Pregnant women and pregnant veterinary staff should avoid handling this medication or the bodily fluids/waste of treated animals. If a nursing dam requires treatment, milk replacer should be used as the drug inhibits DNA/RNA synthesis.
Drug interactions
- Leucovorin: May increase the gastrointestinal toxic effects of 5-FU
- Cimetidine: Inhibits the metabolism of 5-FU, potentially increasing toxicity
- Carboplatin: Synergism in human oncology, but combination may be more toxic in small animals
- Methotrexate: Synergistic if administered before 5-FU; antagonistic if administered afterwards
- Vincristine: Increases the cytotoxicity of 5-FU
Frequently asked questions
What is Fluorouracil used for in pets?
> **CRITICAL WARNING FOR CAT OWNERS**: Human topical creams containing 5-FU (used for skin lesions) are **highly toxic and frequently fatal** to cats. Even a tiny lick or walking across a treated area and grooming can cause death. Keep all such medications strictly out of reach. * **Serious Side Effects**: Understand that this is a potent chemotherapy drug. It can cause severe gastrointestinal upset, bone marrow suppression (increasing the risk of infection), and neurological signs. * **Monitoring**: Strict adherence to follow-up appointments for bloodwork (CBCs) is mandatory to ensure your pet's white blood cell counts remain safe. * **Safety at Home**: Because this drug is a known teratogen (can cause birth defects) and is excreted in bodily fluids, pregnant women, nursing mothers, and children should avoid handling the medication or the pet's waste (urine/feces) for several days post-treatment. Wear gloves when cleaning up accidents.
Is Fluorouracil safe for my pet?
Dosage must be set by your veterinarian for your specific pet. Never give human medication or another pet’s prescription without veterinary guidance.
This article is for general education and is not a substitute for professional veterinary advice. If your pet is unwell, please consult a veterinarian.
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