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November 2023

Side Effects of Methimazole in Cats

Sunday, February 2, 2020
posted by Jim Murphy

Methimazole is the choice drug used to treat hyperthyroidism is both humans and pets. Most tolerate the drug well but some have serious side effects. My cat Atlantis was recently diagnosed with hyperthyroid disease and kidney disease and has been put on Methimazole. He didn’t tolerate the pill too well so now he’s on the transdermal gel which I rub in his ears. has compiled a list of all side effects of Methimazole. Atlantis lost his appetite but is eating again. He is a little sluggish and may be depressed. This is another side effect of the drug.

The Side Effects of Methimazole for Cats

Methimazole for cats is the same as the drug used to treat humans with hyperthyroid disease. This is the same reason that veterinarians prescribe methimazole for their feline patients. Hyperthyroid disease is a common ailment of older cats. You can administer cat medication for hyperthyroid disease orally or topically. Methimazole is now the preferred course of treatment for hyperthyroid disease by most veterinarians because of its low incidence of side effects. There are a few side effects that you should be aware of, however.

The common side effects include:

Loss of Appetite

Methimazole pills are very bitter in taste. The bitter taste may lead cats to quit eating food. In most cases you can avoid the loss of appetite by disguising the pill in food to minimize the poor taste.


As with many medications, vomiting is often a side effect of methimazole when used to treat cat hyperthyroid disease. This is often due to the taste of the pill. Disguising the pill in food can help your cat avoid this side effect.


Methimazole treatment for cat thyroid issues can cause depression in some cats. This is a side effect you will have to watch for. If this side effect develops then you will need to have your veterinarian adjust your cat’s medication.

Disorders of the Blood

Approximately 15% of cats treated with methimazole for overactive cat hormones of the thyroid will develop some from of blood disorder. The most common is leucopenia, which is a low white blood cell count. Low white blood cell counts impact the cat’s immune system and leave them prone to other diseases.

Autoimmune Disease

Most cats treated for hyperthyroid disease with methimazole for six months or longer will develop an autoimmune disease. This happens in about 50% of cats.

Excessive Scratching is a Rare Side Effect

Some cats will self mutilate their neck and face because the methimazole will cause excessive itching. This will develop in the first two weeks if it is going to develop in your cat.

Bleeding is a Rare Side Effect

Some cats will also begin to bleed due to the methimazole. This side effect is rare and you will notice it quickly. Cats with this reaction should discontinue their use of methimazole to treat the hyperthyroid disease.

Side effects from methimazole use in cats are minimal. Most cats respond well to methimazole treatment. There is the possibility that treating the hyperthyroid disease will reveal other issues such as kidney failure or other disorders. Hyperthyroid disease can disguise disorders that only come to the surface after a few months of treatment and further blood testing.

If there are no further complications within the cat, stabilizing the thyroid hormones with the use of methimazole can make permanent treatment possible. Radiation treatment for cats with hyperthyroid disease is possible if a cat does not develop side effects from methimazole treatment and after the thyroid disease reaches a manageable level.

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