Is Methimazole Safe for Cats?

Methimazole is a prescription medication used to treat hyperthyroidism, often in conjunction with surgery. While it can be very effective in lowering thyroid hormone levels, there are significant risks that are not disclosed to your cat. But can methimazole harm cats?

Methimazole Killed My Cat

“After reading the posts about Methimazole, I thought that I should share my story. My cat developed hyperthyroidism about two years ago and was prescribed methimazole. After a couple of months on the drug, he began to lose weight rapidly, despite eating constantly. He also became very withdrawn and would hide all day. The vet told me that this was not uncommon and that he would eventually adjust to the medication. However, his symptoms worsened, so the vet increased the dosage. At this point, I decided to look up methimazole on the internet. What I found scared me – there were hundreds of posts just like mine! Many people reported that their cats had died from liver failure or blood disorders after being on methimazole for only a few weeks. Others said that their cats never adjusted to the drug and remained withdrawn and miserable for months before they finally succumbed to the disease (which is what happened to my cat).”

“My cat was diagnosed with hyperthyroidism. She started on methimazole and it killed her. She was a rescue cat, very old and I adopted her in the fall of 2018. I had no idea that the medicine would put my cat to sleep. My vet told me to give it to her twice a day for the rest of her life and I would have to bring her in every month for blood tests to check her blood levels to make sure she did not get too much or too little methimazole. After a few months on the medicine, my cat was getting worse! She lost more weight, her fur was falling out and she became very lethargic.”

“Methimazole is no joke. I had to put down my cat today because of it. She was on it for only a month. Her name was Roxy and she was my best friend in the whole world. She was so sweet and affectionate, I loved her so much. If you have a cat that has been prescribed methimazole for hyperthyroidism, please be careful with it!”

Methimazole killed my cat?

While Methimazole is typically considered safer than other treatments, it does have some side effects that may be serious or even death if left untreated. The most common side effects are vomiting, diarrhea, loss of appetite, and lethargy.

The most serious side effect is an allergic reaction to the medication. Signs of an allergic reaction include facial swelling, hives, scratching, sudden onset of diarrhea, vomiting, shock, seizures, pale gums, and rapid breathing.

If your cat shows signs of an allergic reaction at any point during therapy, contact your veterinarian immediately.

Do not give this medication if your pet has liver disease or low blood counts (anemia). If you are unsure about the health of your pet, contact your veterinarian before giving this medication.

What happens if a cat takes too much methimazole?

Like any medication, it’s possible for a cat to be accidentally given too much methimazole.

The most common clinical signs of an overdose or intoxication with methimazole are vomiting, diarrhea, lethargy, and loss of appetite. Many cats can become anemic after long-term use of methimazole.

There have also been reports of liver disease and kidney disease associated with the long-term use of methimazole. Signs of kidney disease include increased drinking, increased urination, decreased appetite, and weight loss. Signs of liver disease include jaundice (yellowing of the whites of the eyes), abdominal swelling (ascites), increased drinking, increased urination, decreased appetite, and weight loss.

If you think your cat has been exposed to too much methimazole either because he/she ate too many pills or the dose was miscalculated by your veterinarian it is important to bring your cat into a veterinary clinic immediately if you notice any clinical signs.

How long can a cat live on methimazole?

Methimazole is the active ingredient in Tapazole, a medication that helps to lower thyroid hormone levels in cats with hyperthyroidism. Cats diagnosed with this disease typically live 2 to 7 years after treatment begins, depending on the severity of their disease and how well it is managed.

How fast does methimazole work in cats?

It takes several weeks for methimazole to reduce blood thyroid hormone levels to normal. Typically, it takes at least four weeks. Cats with very high thyroid hormone levels may require six to eight weeks. If your cat’s thyroid hormone level is still too high after this time, the dose of methimazole may need to be increased.

Alternative to methimazole for cats

Propylthiouracil (PTU) is the alternative medication that is commonly used in cats when methimazole is ineffective or causes a reaction. It has been administered to cats for hyperthyroidism since it was first introduced in the 1950s. PTU works by reducing the production of thyroid hormone, just like methimazole. Carbimazole, which is not currently available in the United States, works similarly to methimazole and is a carbethoxy derivative of methimazole.

Though both PTU and methimazole are FDA-approved medications for treating hyperthyroidism in cats, it’s important to note that they have not been tested on pregnant felines and should not be used by them. Do not use these medications if your cat is pregnant unless instructed by your veterinarian.

PTU and carbimazole can cause side effects, including vomiting, anorexia, and liver disease. Carbimazole also has been associated with bone marrow suppression. This condition can be life-threatening if left untreated. While these drugs do carry some risk, serious side effects are rare in cats when the drugs are used appropriately.

While both medications have a wide margin of safety and are generally well tolerated by most cats, there may be reasons why your veterinarian recommends one drug over the other.

What is the best food to feed a cat with hyperthyroidism?

Cats with hyperthyroidism should be fed canned food or raw diets that contain little to no fruits, vegetables, or grains.

If the cat will not eat canned food, then raw meat grinds are an option. If you cannot feed raw, then try a high-quality dry diet made of animal-based protein, with limited carbohydrates.

Under the guidance of your veterinarian, you may be able to order a therapeutic prescription diet from your veterinarian’s office to help manage your cat’s disease.

When to put a cat to sleep with hyperthyroidism

If your cat has hyperthyroidism and is showing signs of pain or discomfort, such as:

  • trouble breathing or swallowing
  • lethargy
  • weakness
  • hunched posture
  • diarrhea
  • constipation
  • vomiting
  • drooling excessively
  • seizures
  • excessive panting
  • dilated pupils
  • abdominal swelling with fluid build-up in the abdomen (called ascites)

Hyperthyroidism is a serious condition that must be diagnosed and treated as quickly as possible. If left untreated, hyperthyroidism can lead to heart failure and other health problems.

If your cat has been diagnosed with hyperthyroidism, the next step will be to determine if they need medication or surgery to treat it. In some cases, medication may not be necessary at all; in others, surgery may be required.

Your veterinarian will determine whether your cat needs treatment based on their age and overall health. For example, older cats often have more difficulty healing from procedures like surgery than younger cats do. If your cat is elderly or has other health problems, you may want to consider putting them down rather than having them undergo a potentially risky medical procedure.

Positive reviews of Methimazole

“My cat has been on this for about a year now and it seems to be working well. He’s stabilizing and not getting worse. I am concerned about the long-term effects of having him on this drug though as I’ve read that it can cause liver damage.”

“It works great! My cat’s fur is growing back and she is her old self again.”

“This was the first drug that was used on my cat and it worked great. My cat has been on this for 2 years and I have never had a problem.”

“My cat was diagnosed with hyperthyroidism and prescribed methimazole twice a day. He is very compliant with his medication and takes it without any issues. His thyroid levels have returned to normal and he has regained the weight that he had lost.”

“My 12-year-old cat has hyperthyroidism that we recently discovered after she started losing weight like crazy. My vet prescribed this medication and my cat seems to tolerate it well. I cut up a tiny piece of food, put it in her mouth, then put the pill on top of the food and she swallows it down fine. She is already gaining weight back after being on this med for only a week so I think the med is working well!”

“This medication was prescribed by our veterinarian for our 13-year-old cat who was suffering from hyperthyroidism. She was given a diagnosis 4 years ago but seemed to be doing pretty well after being treated with Methimazole.”

“Methimazole is a good medication for hyperthyroidism in cats. I used it on my cat and it worked really well. It really brought down his thyroid levels. I gave him the thyroid pills twice a day and he seemed to not mind that at all. He ate them right up without any problem. I did find out that the side effect of this medication is that it can cause liver problems. So I had to watch my cat very closely every few months to make sure his liver enzymes didn’t get too high. For a while, he was taking an additional pill to protect his liver but eventually, it wasn’t necessary anymore. This medication is probably the most common way that vets treat hyperthyroidism in cats, so if your vet recommends this, then you should go for it.”

Conclusion of Methimazole for cats

In conclusion, methimazole is a drug that can be used to treat hyperthyroidism in cats. The initial dose is 2.5-5mg/day, and it can be divided into twice a day. It should be given for at least 6 weeks before deciding whether the drug is working.

The side effects of the drug include vomiting, diarrhea, anorexia, and lethargy. The most serious side effect is agranulocytosis, which is a decrease in white blood cell count. If your cat starts vomiting or has diarrhea after starting the medication, call your vet. These are usually mild problems but can sometimes lead to more serious problems.

Monitor your cat for lethargy and decrease in appetite for the first 3 months of therapy. These symptoms may not be related to the medication but should be reported to your vet if they occur so that they can be evaluated.

If there are any changes in your cat’s behavior or appetite, or if you have any questions about this medication or how it is affecting your cat, please don’t hesitate to call your vet!

Hyperthyroid Cats and Transdermal Methimazole

Hannah Elizabeth is an English animal behavior author, having written for several online publications. With a degree in Animal Behaviour and over a decade of practical animal husbandry experience, Hannah's articles cover everything from pet care to wildlife conservation. When she isn't creating content for blog posts, Hannah enjoys long walks with her Rottweiler cross Senna, reading fantasy novels and breeding aquarium shrimp.

7 Responses

  1. In 2008, I lost my 18.5 year old female cat to methimazole. She was diagnosed with hyperthyroidism late that March, started the medication around April 1st and was dead August 8th. I begged her original vet to help her and was shocked by how we were treated. I went to a cats only vet to try to get an alternate medication and was given a run around that cost my cat’s life. It was like losing a child.

    Now in mid-May 2022, my 17 year old male cat that I inherited from a dear friend has just been diagnosed with hyperthyroidism and the vet is recommending methimazole. My boy is completely healthy–remarkably so for his age, much like my girl was–except for his thyroid disease. I am completely torn about what to do. I feel like it’s deja vu.

  2. My 19-year-old cat also was gone within about a month of starting a low dose of methimazole I too feel like I lost my
    child, it is an unbearable pain. I don’t think the methimazole is exactly what did it because she had heart failure but I do think it contributed as her liver values went up like 5 times after starting it. I avoided methimazole for a year for my cat and tried every supplement and herbal medicine I could find as her t4 was only slightly elevated. It doubled on blood draw in March and she was not acting normal so I decided it was time to try the medication. I don’t know if you believe in holistic medicine but maybe you would be interested in finding a holistic/integrative vet and seeing if they have any other options for your other cat.

  3. My sweet baby Dolly just died last night. She was diagnosed not quite a month ago with hyperthyroidism and the vet prescribed methimazole. I have taken in approximately 50 homeless cats throughout my lifetime, but had been with me from her birth, as her mamma was homeless and pregnant. She was doing great after taking the meds for 10 days and eating like a trooper. Took her to the vet, he saw how good she was doing, then said bring her back at one month. Then a couple days ago, she wasn’t eating again. I wasn’t too worried because I saw that was a side effect when I researched it on the internet. However the vet never mentioned to me anything about any side effects at all, or that this medicine quite often kills! I went to pick up a prescription yesterday on my lunch hour, but didn’t take her in, but now I wish I had. The girl mixed up the liquid for me in a bottle but I noticed it was only half as full as the last time. I gave Dolly her meds about 7:30 and within an hour she was wobbling her head and could barely hold it up. Her breathing was really hard, then she had a seizure and her limbs locked up, then let out the loudest horrific cry I have heard in my lifetime from any cat, despite her quiet demeanor and tiny polite meows. I think at that point she had a heart attack because she coughed three times and had a tiny pulse and barely breathing. I called the vet to discuss what happened and to let them know so it doesn’t happen to another pet owner. I left a message with the receptionist at 7:30 this morning and haven’t heard back and it’s 3 o’clock. I am not trying to get anyone in trouble, because I don’t know if the girl made it too strong or if I should have just taken her off of it a couple days ago when she stopped eating. Please, everyone, this is a dangerous drug and the vets should advise you of this. You should be explained of all side effects and take them off of it or see your vet the very second you see any change at all. In my opinion, this medication should only be administered at a vet’s office with doctors available 24 hours a day to monitor them, until your cat has been taking it for 2 or 3 months. Anything could happen at any time, and tests should be run sooner to see if there are problems early on. I don’t want anyone to ever have to go through what i went through last night. Dolly didn’t deserve that. I was her whole world, she knew no one else to count on but me to help her. But I let her down. I should have taken her off this, and took her back for testing even though the vet said bring her back in a month. I let her down and she trusted me and put her faith in me.

    1. My baby Bear died today of kidney failure. I believe Methimazole caused the condition. The vet started him on Methimazole on May 24 for hyperthyroidism. Before starting Bear on the medication, all bloodwork taken (other than the T4) was in the good range. The vet told me to look out for skin irritation allergy only. She did not indicate that Methimazole could cause kidney failure or any other side effect. She did not offer to start Bear conservatively on Hill’s thyroid food, which I discovered only last night while reading a scholarly article has an efficacy of 75 percent and a second article stating 86 percent effectiveness at treating cats with hyperthyroidism.
      So within eight weeks of treatment with Methimazole, Bear regularly vomited, was lethargic and obviously was not eating enough as he had lost 2 lbs from the 24th of May. Last week I rang the vet and told her Bear felt skinnier since starting the medication, and he was not eating properly and not acting himself. I even asked her about the Hills thyroid diet, which at that time I had not fully researched. My vet told me that if we had not been able to pill him she might have turned to that treatment, but not even before the topical Methimazole compound. I am so angry at myself for wholey trusting my vet and not doing more research on Bear’s condition and treatment options. I too feel I let my baby down, and caused him immense suffering. In less than two months, Bear’s creatinine and Bun had shot up to dangerous levels and his gums were white due to anemia and he was severely emaciated. During the night Bear could hardly stand up, he only drank small amounts of water, periodically moaned and howled, and experienced incontinence. Bear was in pain and dying, and I could do nothing to help him. Though I cannot prove it, I believe that Methimazole caused Bear to acquire kidney failure.

  4. Methimazole Recall

    I had a feeling my cats hyperthyroid medication is what killed her. This week I received confirmation that my theory may be correct. A compensation group reach out to me.

    Without naming any pharmaceutical companies yet – a compensation firm reached out to me about my cats medication. They are collecting data to see what adverse effects are out there from distributed defective methimazole.

    My cat rapidly declined on this medication. She was diagnosed with hyperthyroidism but then died a couple months later of liver failure.

    Other than all the horrific details, I am reaching out and doing my own research. This lawsuit is probably going to take awhile since they are waiting on other customers responses. I am making this post to see if anyone else’s animal got sick or died on methimazole this year 2022?

  5. My 14-year-old, Daisy, got worse on methimazole and was dead within 4 months. Hyperthyroidism caused her blood pressure to skyrocket causing blindness. That is when vets prescribed methimazole 2x a day. At first, she seemed okay and despite being blind she got around the house okay. But Daisy’s appetite slowly decreased until I had to try to feed her every 2 hours. Her gums were pale pink, and edge of tongue was white. Her kidney numbers increased, red blood cell count decreased yet the vet did not offer an alternative. She went from 8 lbs to 5lbs when we finally put her down 4 months later. At the end she had no appetite, was lethargic, anemic, nauseous, had diarrhea, slept on the bathroom floor in an awkward position, could hardly walk, and sometimes peed where she lay, then I think she had a stroke. In the last weeks, I stopped her meds because she was so sick but likely her BP went up again causing the stroke. Poor kitty, I feel guilty now having given her meds that contributed to her death.

  6. We lost our dear Yoda last night to methimazole. It caused autoimmune hemolytic anemia only after 3 weeks. It is a terrible drug. Yoda was 12 years old and in good health except he developed mild hyperthyroidism and was started on this drug 2 tabs a day. He never really adjusted to it and became lethargic and the vet reduced the dose, the next day he couldn’t walk, and back to the vet for the 3rd time in 3 weeks. He suddenly became short of breath and died. If your cat is taking this drug stop it immediately if he is showing any type of reaction.

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