Methimazole Killed My Cat?

When I searched online ”Methimazole killed my cat”, I encountered the following heart-breaking story in a forum.

In March 2021, our cat Smokey was diagnosed with hyperthyroidism and put on methimazole. He was initially doing well on the medication and his thyroid levels were normalizing. However, after about a month on the drug, he showed signs of liver damage. His appetite decreased, and he became more lethargic. We took him back to the vet, and they did blood tests which showed that his liver enzymes were elevated. The vet told us to stop giving him the methimazole and start giving him Denamarin, a supplement that helps protect the liver.

We followed the vet’s instructions and Smokey seemed to improve for a while, but then his condition rapidly deteriorated. He stopped eating altogether and became very weak. We took him back to the vet, and they did more blood work which showed that his liver was failing. There was nothing more they could do for him and we had to decide to euthanize him. It has now been over a year since we lost Smokey and we still miss him dearly. I cannot help but think that if we had not put him on methimazole, he would still be with us today. I would urge anyone who is considering this medication for their cat to do their research and be aware of the potential risks before deciding”.

This is truly a sad story. But can Methimazole kill a cat? We will answer this question along with other things related to Methimazole and cats. Let’s start with the most important question:

Can Methimazole Hurt My Cat?

Methimazole is a medication that is used to treat hyperthyroidism in cats. It works by inhibiting the production of thyroid hormone. Methimazole can be given orally or topically, and is typically given twice daily.

While methimazole is generally safe and effective, there are some potential side effects that you should be aware of. The most common side effect of methimazole is vomiting, which usually goes away after a few days of treatment. Other potential side effects include diarrhea, loss of appetite, and lethargy.

In rare cases, methimazole can cause more serious side effects such as liver damage or blood disorders. If you notice any unusual symptoms in your cat while they are taking methimazole, please contact your veterinarian immediately.

What are the Symptoms of Too Much Methimazole in Cats?

If your cat is taking methimazole (Tapazole) for hyperthyroidism, it’s important to be aware of the potential side effects. The most common side effect of methimazole is vomiting, which can occur within the first few days of treatment. Other potential side effects include:

– Diarrhea

– Loss of appetite

– Weight loss

– lethargy

– Increased thirst and urination

– Difficulty breathing

– Skin problems (e.g., hair loss, itching)

If you notice any of these symptoms in your cat, please contact your veterinarian immediately as they may show a serious problem.

What Happens If I Stop Giving My Cat Methimazole?

If you stop giving your cat methimazole, the level of thyroid hormone in their blood will rise. This can cause several symptoms, including weight gain, increased appetite, panting, increased thirst and urination, restlessness and hyperactivity. Sometimes it can also lead to heart problems.

If you are concerned about your cat’s health, it is best to speak to your vet about the best course of action.

Is There an Alternative to Methimazole for Cats?

Yes, there are a few alternatives to methimazole for cats. The most common alternative is levothyroxine, which is a synthetic form of the hormone thyroxine. Levothyroxine is generally well-tolerated by cats and can be given either orally or by injection.

Other potential alternatives include radioiodine therapy and surgery to remove the affected thyroid gland(s).

When to Put a Cat to Sleep With Hyperthyroidism?

As a cat owner, it’s important to be aware of the signs and symptoms of hyperthyroidism, as this condition can be very serious. If you think your cat may suffer from hyperthyroidism, it’s important to take them to the vet for a diagnosis. There are many treatment options for hyperthyroidism in cats, but unfortunately not all of them are successful.

Sometimes, the only option may be to put your cat to sleep. This is a difficult decision to make, but if your cat is suffering and there is no hope for recovery, euthanasia may be the best option. If you’re facing this decision, it’s important to talk to your vet about all of your options.

They can help you decide whether euthanasia is the right choice for your cat.

Symptoms of Too Much Methimazole in Cats

The most common symptom of too much methimazole in cats is vomiting. Other symptoms include diarrhea, loss of appetite, and lethargy. If your cat is displaying any of these symptoms, contact your veterinarian right away.

They will probably need to adjust the dose of methimazole or switch to another treatment option.

What If My Cat Misses a Dose of Methimazole?

If your cat misses a dose of methimazole, it is important to give the next dose as soon as possible. If it is over 12 hours late, skip the missed dose and continue with the regular schedule. It is also important to call your veterinarian if your cat misses a dose, as they may need to adjust the dosage or frequency.

Find Out Whether Other Things That Can Kill Your Cat


Methimazole is a medication used to treat hyperthyroidism in cats. It works by inhibiting the production of thyroid hormone. While it is an effective treatment for many cats, some may experience side effects from taking too much methimazole.

If you stick to the right dosage (2.5 to 5 mg/cat administered in 2 divided doses), it’s highly unlikely your cat will suffer any side effects.

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Dr Harunur Rashid (Harun) is a Doctor of Veterinary Medicine who has five years of experience in large pet animal medicine. He worked as a livestock officer for two years in an NGO, and since then he has been practicing pet animals medicine privately. He holds an MS in Pharmacology from Bangladesh Agricultural University and a DVM from the same institution.