Methimazole Allergy in Cats

Methimazole Allergy in Cats - Symptoms, Causes, Diagnosis, Treatment, Recovery, Management, Cost

What is Methimazole Allergy?

Because of the seriousness of the disease (hyperthyroidism), the side effects of the drug (methimazole) have to be weighed against the benefits. Sometimes the side effects will dissipate within a few weeks or can be treated with a different medication to lessen the effects. However, if your cat has any serious side effects that have to do with the liver such as yellow gums, skin, and eyes, vomiting, lethargy, skin lesions, and swollen lymph nodes an evaluation of the use of this medication must be done. It is important that your veterinarian do a blood test to determine the lowest amount of methimazole possible before starting the medication. In addition, blood tests to keep track of liver and kidney enzymes, thyroid hormones, and other abnormalities should be done on a regular basis.

Methimazole is a drug prescribed to treat hyperthyroidism in cats that subdues the processing of thyroid hormones. It is not specifically regulated for animal use but has been accepted for veterinary use by the FDA. This drug should only be used for a short time in cats because of the risk of serious allergy symptoms such as swelling of the face, scratching, and hives, which can lead to seizures, shock, and even coma. Other complications include appetite loss, vomiting, weakness, and liver damage. Some of the less common side effects include anemia, lymph node swelling, abnormal bleeding, and jaundice. If your cat has any of these symptoms when taking methimazole, you need to see your veterinarian right away.

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Symptoms of Methimazole Allergy in Cats

Since some of these symptoms (vomiting, weight loss, weakness) are also signs of hyperthyroidism, you may not notice them right away. The most common signs you should watch for include:

  • Vomiting or regurgitating undigested food
  • No appetite (refusing to eat)
  • Rash
  • Swelling of the face
  • Excessive scratching (especially head, neck, and face)
  • Bleeding disorders
  • Cold limbs
  • Shock
  • Seizures
  • Liver failure (yellowing of the skin, eyes, and gums, vomiting, lesions anywhere on the body)
  • Coma
  • Death


Methimazole is a prescription drug made for humans that reduces the amount of thyroid hormones (T3 and T4) your cat’s body produces. It has been accepted for use in cats with hyperthyroidism. This drug also goes by other names such as:

  • Felimazole
  • Tapazole

Causes of Methimazole Allergy in Cats

The cause of methimazole allergy is the consumption of methimazole. In some cases, the body attacks the drug and this causes histamines to be released, which creates the symptoms such as vomiting, diarrhea, swelling of the face, scratching, hives, shock, and even coma.

Diagnosis of Methimazole Allergy in Cats

The allergy symptoms of methimazole are usually pretty straightforward, but the veterinarian will probably need to rule out other issues before making a diagnosis. Because the drug has many side effects besides the allergy symptoms, the veterinarian may decide to stop the drug immediately anyway. However, a comprehensive physical examination, blood tests, urinalysis, and radiographs will all be needed to ensure there are no underlying conditions besides the hyperthyroidism.

Treatment of Methimazole Allergy in Cats

The treatment for methimazole allergy is the immediate stoppage of the drug. Because hyperthyroidism is a life-threatening condition, other treatment will need to be decided on right away. There are two other choices that veterinarians use for hyperthyroidism, which are radioiodine or surgery.

Radioiodine (I-131)

Radioiodine is a one-time injection that is effective and safe. Because it is radiation, your cat will have to stay in the hospital for several days (by law), but there are no other medications or injections required. It does not damage any other organs or tissues, will not damage the healthy parts of the thyroid glands, and has no serious side effects like the other drugs. The only drawback is the cost, which averages about $1,000, depending on where it is done. Unfortunately, some cats are unable to get the medication because they are not healthy enough. Radioiodine treatment is only able to be done at certain hospitals that have permits for radioisotope use.

Surgery (Thyroidectomy)

Removal of the thyroid glands is a procedure that is usually only done on cats that are relatively healthy (besides the hyperthyroidism) and under 15 years old. Most often, both thyroid glands are removed, but if the veterinarian believes that one is healthy, it will be left to do its job. It is an effective surgery that is done often, but the parathyroid glands are sometimes wrapped around the thyroid glands, making it a difficult process.

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Recovery of Methimazole Allergy in Cats

Hyperthyroidism is a fatal condition that can cause organ failure and death quickly without treatment. If your cat cannot be treated with methimazole, radioiodine will be used, if possible. However, surgery may be the best choice if your cat has allergies.

Methimazole Allergy Questions and Advice from Veterinary Professionals


DMH Feline



Sixteen Years


44 found this helpful


44 found this helpful

My pet has the following symptoms:
My 16- year old female was diagnosed with hyperthyroidism. After taking methimazole for 5 weeks, we discovered she is allergic to it. Excessive itching and scratching of her lips, face and ears. She is now off the methimazole and being treated for the infection, itching, nausea etc. I've heard there is an ointment for treatment. My question is would this cause the same allergic reaction? I don't think surgery is an option because she is 16. Nor is radiation due to her having to be quarantined for so long. I know there is a special diet food. We are a multi-cat household.

July 16, 2020

Answered by Jessica N. DVM

44 Recommendations

Hello- Thank you for your question. I do think that the transdermal methimazole will likely cause the same issue for your cat. The active ingredient is the same. If surgery or I 131 therapy Are not options for your cat I would recommend trying the specialized diet. Even though you are a multi cat household I do think the diet could work for you. You would need to supplement your other cats diets with some over-the-counter food, but as long as you give that to them separately and your hyperthyroid cat is unable to consume it then it should work for your hyperthyroid kitty as well as your normal non-hyperthyroid cats.

July 16, 2020

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16 Years


22 found this helpful


22 found this helpful

My pet has the following symptoms:
Tachycardia,Slight Heart Murmur
My 16 y/o male cat was diagnosed with hyperthyroidism and prescribed felimazole 2.5mg twice a day-quickly put weight back on and responded well over about ten days.i noticed he had a twitch which increased to jerks over several days from about 1 1/2 hours after his first tablet.informed the vet but they didn't see a connection.told to take him off it for a couple of days-then to restart,with the thinking that if the jerks/twitches stopped and then restarted when meds recommenced, was meds.just before the two days were over my cat then had a seizure which he had never had before.kept off meds for about a week then put back on by original vet who said had never heard as a had slight twitches but virtually stopped-as soon as he went back on the felimazole tablets he started twitching and jerking again-again my vet said to keep him on the medication.again I took him off the meds after ten days-within three days he had another seizure.both at night in his sleep(the one time previously he had not really twitched on the meds), both when taken off the meds.since being off the meds and having the seizure the twitches and jerks had stopped.he has now been off the felimazole for ten days and needs treatment.i have started him on y/d food-hills-and was told by the emergency vet he saw after the second seizure that he shouldn't be on felimazole as there was a bit of a coincidence this had occurred again-so probably a link,also that my vet needed to refer back to manufacturer on have seen my vet today to look at treatment options and he has suggested the transdermal gel-0.1ml/twice concern is that this is the same medication and will have the same impact-ie vet again doesn't seem too concerned and has also suggested surgery-but obviously not until my cat's heart rate is lowered.I wonder if you might be able to advise if this gel is likely to have the same reaction as the Felimazole tablets please?my vet suggested that the likelihood was a reaction to something in the tablet-but as they appear to contain justthiamazole(methiamazole)2.5mg as active substance with excipients titanium dioxide and erythrosine could these ingredients have caused the seizures?-doesn't seem likely but I'm not the vet.

Aug. 20, 2018

Answered by Dr. Michele K. DVM

22 Recommendations

I would be hesitant to use the transdermal gel for your cat, as well. You may have Y/D as the only treatment option for him, other than surgery. Y/D can be quite effective in controlling thyroid disease, and may work well for him. You just need to make sure that that is the only food that he has access to. I hope that goes well for him.

Aug. 20, 2018

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