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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.

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.

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

11 Years
Moderate condition
0 found helpful
Moderate condition

Has Symptoms


My 11yo DSH neutered male cat was diagnosed with hyperthyroidism. We began a Methimazole pill regimen (2.5 mg twice a day). After four days, he began vomiting regularly. The vomiting was not concurrent with the pilling or with feedings. Generally, the vomiting occurred between 4-6 hours after his pill.

He also became jaundiced, then stopped eating completely. The Methimazole pills were discontinued immediately. An exam revealed probable pancreatitis (not fatty liver).

After he recovered, we began transdermal Methimazole, in roughly the same 2.5mg concentration, applied twice a day (alternating ears with each application). After 9 doses, his vomiting resumed. As before, it occurred 4-6 hours after the Methimazole application.

He is now on Hills y/d food, but has shown a lack of interest in the food (dry kibble) at times. He has not stopped eating, but it's clear he does not like the y/d food.

I am looking for other treatment options.

Dr. Michele King, DVM
Dr. Michele King, DVM
479 Recommendations
Thank you for your email. Apollo may benefit from radioactive iodine treatment - that is one option for treating hyperthyroidism in cats, but it is only beneficial if he has adequate kidney function. Your veterinarian can discuss this option with you, whether it is possible, and where it may be performed, as only specialty hospitals tend to administer the treatment.

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13 Years
Moderate condition
0 found helpful
Moderate condition

Medication Used


My cat has been on tapazole for about 5 years now with no allergy symptoms about three weeks ago he started being really hyper and scratching a lot! He doesn't go outside he is an indoor cat recently had a health check up could his new dry food or tapazole causing this? Thanks

Dr. Callum Turner, DVM
Dr. Callum Turner, DVM
1978 Recommendations
Whilst it is possible that the medication is causing this response as cats may develop allergies to medicines and other substances overtime; if there is a correlation of the symptoms presenting and the changing of the food I would look more into that. Try to change the food back to the old food to see if there is a decrease in symptoms, otherwise try a hypoallergenic diet to see if that helps; if the issue continues visit your Veterinarian. Regards Dr Callum Turner DVM

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