Felimazole tablets | Felimazole for Cats | Felimazole Side Effects

What do Felimazole tablets look like?

Felimazole comes in small, brightly coloured, sugar-coated tablets. There are 2.5mg tablets which are pink, and 5mg tablets which are orange. They are designed to be given directly into the mouth and swallowed whole. However this is very difficult with some cats and lots of cats take them in food. The tablets are very small and appear to be odourless and tasteless to cats, and so if added to tasty, moist food, they are easy enough to give in most cases.

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What is Felimazole used for?

Felimazole tablets are given to control hyperthyroidism. This is a common condition of older cats and is due to over-production of thyroid hormones from the cat’s thyroid glands. A pair of these glands is present in either side of the neck, but they are usually very small and difficult to feel. In hyperthyroidism, one or both glands become very much enlarged, often becoming easy to feel. Felimazole tablets contain the drug thiamazole which suppresses the amount of thyroid hormones released (called thyroxine, often known simply as T4, and triiodothyronine, known as T3). An enlarged gland will not shrink with treatment, but its activity will be controlled.

Is there any need for treatment?

Thyroid hormones tend to speed up the body’s metabolism, so when they are over-produced, the affected cat tends to lose weight despite eating large amounts. Indeed some of these cats become enormously greedy, but still lose weight. They also tend to have a fast heart rate, are overly active for their age, and can suffer from the effects of high blood pressure. They can suffer bleeding and detachment of the retina in the eye, sometimes going blind. They are also more prone to heart and kidney disease. Some cats show other symptoms and can go off their food instead of being greedy, worsening the weight loss. They may be prone to vomiting or diarrhoea, physically weak and subdued. If left untreated, affected cats are unlikely to live as long as they would with treatment, and may well go blind or suffer from other problems.

What are the Felimazole Side Effects?

Mild side effects are not unusual, though the vast majority of cats show none at all, and thrive on treatment. The most common side effects are vomiting, going off food and becoming listless. Occasionally more serious side effects are seen and these can be quite varied. Some cats seem to feel itchy about their face and neck. They then scratch themselves and develop open sores and scabs on the head and neck. This might not happen immediately treatment is started, but will usually be within the first 3 months. Other effects seen more rarely might involve the bone marrow, resulting in anaemia or abnormalities of the white blood cells and immune system. The mild side effects usually settle down after a short while and do not require treatment to be stopped. If more serious side effects are suspected, treatment should be stopped and veterinary attention sought.
It is advised that blood tests are done before treatment and then periodically afterwards to check for potential problems, and also to monitor the levels of thyroid hormones. Normally a low dose is given to start off with, and then increased as needed. Obviously this is all done under the supervision of your vet.
It is suspected that if a woman in early pregnancy takes in any of the active ingredient in Felimazole, it might affect the development of her baby. Clearly this could be very serious, so women should be careful handling the drug if there is any chance they could be pregnant. Also any person can become allergic to the active ingredient. For these reasons it is sensible to wash one’s hands after handling the tablets and not to break or crush them. Also some of the drug is passed out in the cat’s urine, so women who might be pregnant should not handle treated cats’ litter trays, or should wear gloves to do so.

Are there any alternative treatments?

There are two other ways to treat hyperthyroidism in cats. For cats which are not suffering from any heart, kidney or other conditions, treatment with radioactive iodine is often a very good treatment. However this is very expensive and means that cat has to live in strict isolation at the treatment centre for a week or two. The other treatment option is to surgically remove the over active thyroid gland(s). These treatments can provide a permanent cure without the need for tablets in future, though various complications can occur.
Generally speaking Feliimazole medication works well as a treatment for hyperthyroidism in cats. It can just be used for a few weeks prior to surgical removal of diseased thyroids, or if surgery is not suitable, it can be maintained for the rest of the cat’s life. Most cats respond well to long-term medication and live the rest of their lives normally.

Where to get Felimazole

Felimazole can be supplied by your local vet. This might be the most expensive way, but it is easy. However Felimazole is not a cheap drug and can be supplied legally online by places such as VioVet. Many people use online suppliers these days to help with the costs of medication. It must first be prescribed by the vet who is directly responsible for the care of the cat in question. This means that a vet must assess the cat’s condition and then decide, in conjunction with the owner, what treatment is most appropriate. If the decision is to use Felimazole, then the vet is legally obliged to provide a written, signed prescription on request. This can then be forwarded to an internet pharmacy such as VioVet so that the drug can be supplied.

(VioVet use exactly the same Felimazole as your local vet but are able to supply it more cheaply.)

Article written by:
John Cousins BVSc MRCVS - A veterinary surgeon from VioVet - Online Pet Store & Pharmacy.

The article reflects the opinions of the author and should only be used as a guide. It can only be taken in conjunction with advice from your vet. No liability is accepted for accuracy of content or any other matter.