Methimazole for Cats & Vomiting

OK, I'm ready for the pill.
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If your cat's been diagnosed with hyperthyroidism, your vet probably gave you a few treatment options. The medication option consists of giving Kitty methimazole twice a day. While generally well-tolerated, some cats can't stomach the drug -- literally. However, a transdermal gel you place in his ear is a possibility.


Marketed under the brand name Tapazole for people and Felimazole for cats, methimazole inhibits thyroid hormone synthesis. Cats with hyperthyroidism, or too much thyroid hormone, usually have a benign tumor on their thyroid glands, located in the neck. If your older cat constantly begs for food but loses weight, drinks and pees excessively, experiences vomiting and diarrhea, stops grooming himself and significantly increases his level of activity, suspect hyperthyroidism. What you can't see is his high blood pressure and the damage done to his heart or kidneys. The thyroid hormones regulate his metabolism, so most organs are affected.

Side Effects

According to Veterinary Partner, approximately 15 percent of cats experience side effects when taking methimazole, generally consisting of nausea and vomiting. He might also lose his appetite and become depressed. There's also a danger of him becoming anemic. While 85 percent of cats do just fine on the drug, that's not much consolation if your poor nauseous Kitty is one of the 15 percent. Your vet will temporarily discontinue methimazole until your cat's symptoms subside, then restart it at a lower dose. If gradually increased, most cats can tolerate the drug. However, if your cat experiences severe facial itching, a rare side effect of methimazole, it will start up again if the medication is resumed so you'll need an alternative treatment.

Transdermal Gel

Although you can get flavored versions from a compounding pharmacy, methimazole is a bitter-tasting drug. If Kitty throws it up or just won't swallow the pill, ask your vet about the transdermal gel version, pluronic lecithin organogel. While it isn't as efficient a delivery system as the pill, the pill form isn't efficient if you can't get it or keep it down your cat. Side effects are much lower in cats given the gel.


Methimazole is not the only game in town for hyperthyroidism treatment. While it can treat the condition, it can't cure it. Two alternatives to drug therapy can offer a cure. If your cat is healthy enough to undergo general anesthesia, which might not be the case if he's suffering from kidney or heart disease, ask your vet about a thyroidectomy. This surgery to remove affected parts of his thyroid glands usually cures his hyperthyroidism unless pieces of thyroid tissue have migrated to other parts of his body. The treatment of choice for hyperthyroid cats is radioactive iodine therapy, which is a cure. Only certain facilities perform this procedure, which consists of giving your cat a shot of radioactive iodine, which gets rid of abnormal thyroid cells without harming normal ones. Because Kitty is radioactive for several days, he will be in quarantine until the radioactivity drops to safe levels, so you can't visit him. Another possible treatment, which doesn't cure hyperthyroidism involves feeding him a special veterinary diet with a small amount of iodine that normalizes thyroid output. The drawback is that your cat can't eat anything else -- no treats, table scraps or non-medicated cat food.

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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