Methimazole is used to treat hyperthyroidism in cats. It works by blocking the thyroid gland from making thyroid hormone. The effect is temporary. Most cats with hyperthyroidism require lifelong treatment with methimazole unless other, permanent treatment options are pursued. More details on these options are available on the page entitled Hyperthyroidism in Cats.
Methimazole most commonly is administered as an oral pill once or twice daily. Certain pharmacies (called compounding pharmacies) will provide methimazole as a flavored liquid or as a gel that can be rubbed onto the cat’s skin. Some experts question the efficacy of these unconventional forms of methimazole.
The most common side effect of methimazole is gastrointestinal upset (vomiting, diarrhea, or loss of appetite). Rarely, cats taking methimazole may develop intense itching and a rash around the face. Blood cell imbalances are a very rare side effect of methimazole. Contact your veterinarian immediately if you suspect your cat is suffering from side effects.
Cats taking methimazole require regular blood tests as well as monitoring of weight and clinical symptoms. Dosage adjustments and discontinuation of medication should occur only under the supervision of a veterinarian.
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