Major Uses

Methimazole is an anti-thyroid medication used to treat hyperthyroidism in cats. It works by blocking production of T3 and T4, two thyroid hormones. The medication is a popular choice for feline hyperthyroidism treatment due to its low risk of serious side effects and affordability.

Methimazole is for use in cats only. It is not approved by the Food and Drug Administration for use in animals, but it is legally prescribed as an extra-label drug for this purpose.

Common Precautions

Side effects associated with use of methimazole are uncommon and disappear with cessation of the medication. If no side effects occur during the first few months of therapy, the chance of side effects occurring in the future is substantially reduced. When side effects do occur, they tend to be mild. The side effects most commonly associated with use of methimazole in cats include loss of appetite, vomiting, and lethargy. Temporarily stopping the medication and then restarting at a lower dose generally clears up any side effects. Other adverse effects associated with use include bleeding disorders, blood-cell disorders, and liver disease.

Rarely, facial itching may develop that can become severe. This side effect does not usually resolved with continued or repeated use of the medication, and discontinuing methimazole is generally necessary. Additionally, liver failure is a very rare side effect that requires immediate cessation of the medication.

Hyperthyroidism may mask existing kidney disease. The medication should be used cautiously in animals with immune-system disease, liver impairment, or blood disorders.

Methimazole may interact with a number of other medications and supplements, including beta-blockers, benzimidazole antiparasitics, bupropion, digoxin, phenobarbital, theophylline, and warfarin. Methimazole should not be used in animals with a known allergy or hypersensitivity to the drug. Any known or suspected overdose should be reported to a veterinarian right away.


Methimazole should be stored in a tightly-closed container, away from light and moisture, and at room temperature. This medication should not be kept in the bathroom or above the kitchen skin. Methimazole should be stored out of the reach of children and animals.


Methimazole is available as tablets in strengths of 5 mg and 10 mg. The initial dose of methimazole in cats is typically 5 mg per cat, three times each day. If testing shows no improvement in thyroid hormone levels, the dose of methimazole may be slowly increased.

Doses may vary in different species, when the drug is given by a different route or concurrently with other medications, and with regards to a patient's age, breed, and health status. A veterinarian's dosing instructions and/or those printed on the medication label should be followed closely.

If a dose of this medication is skipped, it should be taken as soon as possible. If it is almost time for the next scheduled dose, the missed dose should be skipped and the normal schedule resumed. It is important not to give two doses of this medication at once.

This information is for general reference only and should not be used for the diagnosis or treatment of any condition of your pet. It's intended as a general reference, this information may not include all possible uses, precautions, directions, reactions (including allergic), drug interactions, or withdrawal times. Always consult your local veterinarian and have your pet examined for any advice concerning the diagnosis and treatment of your pet, including which products and doses are most appropriate. Any trademarks are the property of their respective owners. VetDepot is not a pharmacy. All prescription products are dispensed by our Pharmacy Partner. Article last updated 2/2014.