Allergy to Methimazole in Cats

"That facial itching was awful."
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If your cat's been diagnosed with hyperthyroidism, your vet might prescribe methimazole for treatment. Sold under the names Tapazole and Felimazole, most cats tolerate the drug well. However, some cats experience an allergic reaction. If that's the case with your kitty, alternatives exist for hyperthyroidism treatment.


Your cat's thyroid glands regulate much of his metabolism. When they go awry and start producing too much thyroid hormone, all sorts of symptoms appear. Your older cat loses weight although he's constantly crying for food. He might cry out for other reasons -- "loud vocalizations," also known as constant meowing, is another sign of the condition. He's hyperactive, cranky, drinking and peeing a lot, with an unkempt coat and possible hair loss. Your vet diagnoses hyperthyroidism by testing your cat's blood thyroid levels. Methimazole is available in pill and transdermal gel form -- you put the latter in your cat's ear every day.

Allergic Reactions

A small percentage of cats suffer an allergic reaction to methimazole resulting in a skin rash and constant and severe facial itching. Affected cats indulge in incessant self-mutilation. According to, approximately 4 percent of cats administered methimazole experience this allergic reaction. Your vet will tell you to take the cat off the drug and she'll prescribe medication to stop your cat's scratching agony. No more methimazole for Kitty -- you'll need to find an alternative therapy.

Side Effects and Contraindications

While the facial itching is the primary allergic reaction, some cats experience side effects. These side effects generally show up within the first few weeks of treatment. Your vet might discontinue giving your cat the medication for a certain time, then begin again at a lower dose. Common side effects include vomiting and appetite loss, as well as more serious issues such as blood abnormalities, liver failure and bone marrow suppression. Don't give methimazole to cats with kidney, liver or autoimmune diseases, or anemia. Pregnant and nursing cats shouldn't receive methimazole.


If your cat's allergic to methimazole, you have three options. Your vet can prescribe a low iodine cat food, which your cat must eat for the rest of his life. He can't eat anything else -- no treats, table scraps, nada. If he's your only cat, that might not be a problem, but it's not easy in a multi-cat environment. Your vet can perform a thyroidectomy, surgical removal of the thyroid glands. This provides a cure rather than a treatment, but some older cats are too fragile for surgery. The preferred treatment is radioactive iodine therapy, consisting of a simple injection of the radioactive material. The downside for this method is that you'll need to transport your kitty to a veterinary facility providing this specialized treatment, leaving him there for a week or so until he's no longer radioactive. You won't be able to visit him while he's quarantined.

Always check with your veterinarian before changing your pet’s diet, medication, or physical activity routines. This information is not a substitute for a vet’s opinion.

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