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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

Types

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
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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.

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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.

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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.

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Methimazole Allergy Questions and Advice from Veterinary Professionals

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Ask a Vet

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Domestic shorthair Cat

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Sixteen Years

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Rash On Ears

My 16 year old is getting a bad rash, itchiness and redness on her ear where I put her 2.5 ml methimazole transdermal medication. Is there anything I can put on her ears to help this?

Aug. 6, 2020

Owner

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Dr. Gina U. DVM

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0 Recommendations

Hello It sounds like she is having a reaction to the medication. I recommend consulting with your veterinarian about switching to an oral medication until her rash heals. Good luck.

Aug. 6, 2020

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DMH Feline

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Sixteen Years

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Redness

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

Owner

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Jessica N. DVM

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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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mycat

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moggie

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16 Years

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Serious severity

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4 found helpful

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Serious severity

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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 reaction.cat 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 this.now-i have seen my vet today to look at treatment options and he has suggested the transdermal gel-0.1ml/twice day.my concern is that this is the same medication and will have the same impact-ie seizure.my 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

mycat's Owner

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Dr. Michele K. DVM

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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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Courage

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Domestic shorthair

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10 Years

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Mild severity

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Has Symptoms

Face Swelling

My cat Courage has been taking methimazole for seven months. He has gained weight and seems much better. However, his face swells every few days for about twenty minutes and he has to keep his mouth open to breath. I have read that this can be a symptom of an allergy to methimazole, but he had this "face swelling" before he ever took methimazole. Can it be a symptom of hyperthyroidism? I have kept a food diary to see if there is a cause that I can find, but it seems entirely without a pattern. In fact, the swelling seems to disappear quicker if I give him food. I bought him an air purifier and I run it day and night, but it doesn't make a difference. He gets the swelling in the winter when the windows are closed and in the summer when the windows are open (we live in Minnesota where the winters are very cold and snowy and the summers are very hot and humid) I was wondering if it is an allergy and might respond to Zyrtec. Is that safe for cats with hyperthyroidism? Or should I ask my vet for hydroxyzine? Is there any allergy medication that is preferred?

July 13, 2018

Courage's Owner


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2 Recommendations

If the swelling occurred before the start of methimazole, it is not going to be the cause; there are many possible causes for facial swelling and I don’t think it is related to the hyperthyroidism or methimazole; I cannot be sure it is an allergy due to the quick onset and resolution of the swelling in a short time frame without any treatment. You should speak with your Veterinarian to see if there is a specific cause which may be identified. Regards Dr Callum Turner DVM

July 14, 2018

Thank you! Courage is going to the vet soon for a CBC and a new prescription of methimazole.

July 14, 2018

Courage's Owner

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Yanke

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Domestic short

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12 Years

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Serious severity

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My cat had an allergic reaction to the thyroid meds (itchy, facial swelling, lethargy) so we took her off the meds. It's been 1 week and she's doing a lot of vomiting and is still very lethargic. How long would it usually take before they are back to themselves?

June 6, 2018

Yanke's Owner


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8 Recommendations

Any adverse reactions from medications should be handled by your Veterinarian and they should be informed as they may wish to pass the information back to the manufacturer as part of pharmacovigilance and to adjust treatment. Overall it depends on the how long Yanke has been on methimazole, however you should visit your Veterinarian regardless. Regards Dr Callum Turner DVM

June 7, 2018

My cat, age 10, had 2 doses of methimazole, and vet recommended I stop because my cat stopped eating and was clearly worse off. She had been zooming around the house and usually woke me to get her breakfast. She didn't come down for breakfast or treats. That was 3 days ago, and she not only has not recovered, she seems quite a bit worse off. Barely drinking, vomiting what she eats, very lethargic. I am heartbroken. It seems this drug has brought her to death's door.

July 15, 2018

Lynne L.


My kitty past away 2 days ago from allergic reaction to this drug. My heart is broken and I feel I can never trust a vet again to make such decisions. Beware of this drug.

July 7, 2018

Chris-Nancy C.

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Fushi

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American Shorthair

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11 Years

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Critical severity

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Critical severity

Has Symptoms

Pain
Lethargy
Vomiting
Not Eating
Listless
Not Moving

6 weeks ago, I brought my cat in for a check-up. She was playing with her favorite toy and running around the night before. Everything on her bloodwork was fine except a slightly elevated T4, so we ran an FT4, it was elevated as well. I started Methimazole transdermal the next day. Day by day she became listless, I stopped after 1 wk 3 days. She hasn't really eaten since, severely dehydrated, been trying to syringe feed & giver her water. She is now in the hospital, almost on her death bed. Lost 2 lbs in 6 wks. Dr. says he doubts it's methimazole. I don't believe that. :( She is a good doctor & it's not her but the medication. Now her kidneys are bad, her CBC shows severe anemia, she is really sick. I wish I could go back in time. I'll visit her tomorrow morning & hope we can turn it around for her. I'm not ready to let her go. I am absolutely heartbroken.

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Sootie

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Shorthair

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17 Years

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Serious severity

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Serious severity

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Weakness
Weakness, Hind Leg

Have 17 yr old male. Been on methimazole for about 2-3 yrs. Have vet visit regularly and just within the last 6 weeks things dramatically changed for my cat. Vet upped his medication like 30% and cat started severe diarrhea problems. Cut doseage myself and issue seem to resolve. Last wk cat's hind quarters started to get weaker. Took him to vet. He told me cat's older, methimazole not regulating thyroid and surely not helping his kidney disease. Time to euthanize him. Told Vet I wanted to take him home and see what happens. He gave me some metacam for leg weakness. To date cat walking a little better and cut back on methimazole as feel if its not helping why poison cat...Thinking of discontinuing medication and trying herb version instead....Not sure at his old age I want to make his time miserable and today he's walking better.

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Scout

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Tuxedo

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14 Years

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Moderate severity

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Moderate severity

Has Symptoms

Diarrhea
Vomiting
Not Eating
Extreme Weight Loss

I recently discovered that my cat’s medication (tapazole or by it’s generic name Methimazole) has side effects that mimic IBS. in other words; nausea, vomiting and DIRREAH are common side effects for this medication. As tapazole is a human drug you can verify that here at Davis Nursing https://nursing.unboundmedicine.com/nursingcentral/view/Davis-Drug-Guide/51486/all/methIMAzole. As a pet medication you can verify it with the FDA here (it is the animal approved methimazole) https://www.fda.gov/animal-veterinary/animal-health-literacy/hyperthyroidism-cats-theres-fda-approved-drug-treat-it#effects. I brought my cat to his vet because he was having chronic diarrhea. A blood test later and he was diagnosed with hyperthyroidism and put on tapazole which was supposed to stop the diarrhea. It didn’t. My cat began vomiting a little at first and then chronically. His diarrhea became water. My vet gave him antibiotics and Flagyl to no avail. This went on for a year with my vet upping the tapazole dose for his thyroid and ultimately submarining his gastrointestinal system. The last effort to combat the “IBS” issue was a shot of prednisone which did nothing. The end result is that my previously healthy (but with hyperthyroid 12 lb cat now 5 lbs) not eating, diarrhea spewing, vomit spewing and near death’s door cat was actually having side effects from the tapazole for a year! Side effects should have been the first thing my vet considered. Because he certainly didn’t tell me what the side effects were. What he said was that the drug was really safe and had insignificant side effects. I was the one who ultimately correctly diagnosed my cat. He was taken off tapazole immediately and given antibiotics, anti-nausea and appetite enhancing drugs. The first day off of the tapazole his diarrhea improved significantly and with the anti-nausea medication he began to eat slowly at first and then more and more. I don’t know if he is going to make it but I now have hope. So, if anyone out there is experiencing anything similar it is entirely possible that the medication is the cause.

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sootie

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tabby

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16 Years

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Moderate severity

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Moderate severity

Has Symptoms

Diarrhea

I have an older cat who had methimazole increase about 2-3 weeks ago from Vet and since then has had diarrhea and appetite fading slowly but surely each day. I can't make it to Vet for a couple of weeks with working. Should I discontinue Methimazole for a few days and then restart at lower doseage. Noticed my older cat having hind leg weakness the last few days too.. Worried!!

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Sugar

dog-breed-icon

mixed

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16 Years

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Moderate severity

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0 found helpful

pill-rating-filledpill-rating-filledpill-rating-filledpill-rating-filledpill-rating-filled

Moderate severity

Has Symptoms

Lethargy
Weakness
Loss Of Appetite
Hungry But Won'T Eat

My 16 year old was diagnosed back in late November/ early December. The strange thing in her case is that she never had an overactive appetite. Rather she would go through periods of being hungry, but refusing to eat anything that was given to her even though she followed me around and meowed for food. They started her on 5mg dose of methimazole twice a day. About 4-6 weeks later, she went back in to get checked. They said she had gained half a pound and the readings were good, to keep her on the same dosage. She never had any side effects like vomiting or diarrhea. But now she's not eating much of anything. She just lays in bed pretty much. She's still drinking water decent, although not going to the bathroom as much as she was. She seems a little slow at getting up and weak, but it could be from not eating much in the past few days. I'm worried after reading these comments and experiences that it could be the medication. But it had seemed to be working! And she doesn't have any other options with her age/ health. Please help. She's a small cat to begin with and only about 6 lbs. She can't afford to lose more weight. :( She was starting to come back to being her normal self again, and now it seems like she's all of a sudden randomly regressing. Please help!

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