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. 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 so a "pill popper" might be useful.
Like all medication side effects are possible, 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. 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.
In the UK felimazole is a prescription drug and so can only be obtained via your veterinary surgeon, I am not going to write on dose rates as this should be worked out by your veterinary surgeon following blood tests to work out the severity of your cat's disease. Cats on this medication should have blood tests from time to time both to help your vet work out the dose rates and to check for side effects.
1. Hyperthyroidism In The Cat, Symptoms And Diagnosis.