Side effects of tapazole in felines

Written by sharon platz
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Side effects of tapazole in felines
Methimazole (tapazole, felimazole) treats cats with hyperthyroidism. (cat face image by Viktor Korpan from

Tapazole and felimazole are brand names for methimazole. Although tapazole is discussed in veterinary medicine, it is the human form of methimazole, according to Doctors Foster and Smith Pharmacy. Felimazole is the feline form of methimazole, and often prescribed for cats with hyperthyroidism. Methimazole controls hyperthyroidism by blocking the body's increased production of the thyroid hormones T4 and T3. Treatment of hyperthyroidism by methimazole is often preferred over surgery or radiotherapy, writes Mar Vista Animal Medical Center; however, there are side effects that may be harmful to the cat.

Common Side Effects

Mar Vista Vet and Doctors Foster and Smith both report that the most common side effects associated with methimazole (tapazole, felimazole) occur within the first two to three weeks of treatment and are lethargy, loss of appetite and vomiting. Oftentimes, these side effects will work themselves out with continued treatment at a lower dosage. Doctors Foster and Smith warns, however, that if the cat becomes extremely lethargic, develops a fever and bruises or begins to bleed, it should be taken to the vet immediately.

Side effects of tapazole in felines
Lethargy is a common side effect of methimazole. (sleeping cat image by Maciej Syrek from


As with all medications, there is a possibility that the cat will be allergic to methimazole. Signs of an allergic reaction range in severity and include, according to Doctors Foster and Smith, indications of discomfort such as scratching due to hives and facial swelling, diarrhoea, vomiting, and dangerous reactions such as seizures or coma.

Side effects of tapazole in felines
A cat allergic to methimazole might break out in hives. (scratching cat image by Lucid_Exposure from

Bone and Muscular Effects

In fewer than 4 per cent of cats, methimazole can impact white blood cell patterns in the cat's bone marrow, according to Mar Vista Vet. Doctors Foster and Smith also cautions that in rare cases, methimazole use in cats causes myasthenia gravis, a "neuromuscular disease that severely weakens muscles and may cause difficulty swallowing."

Side effects of tapazole in felines
Methimazole can affect white blood cell patterns. (red and white glass image by Vita Vanaga from

Facial Itching

Facial itching is a serious side effect of methimazole. Mar Vista assures this side effect is also only present in 4 per cent of the cats taking methimazole; however, the feline scratches itself to the point of damaging facial tissue and bleeding. Both Mar Vista and Doctors Foster and Smith state felines suffering from facial itching should undergo alternative treatment.

Side effects of tapazole in felines
Cat's claws cause skin damage when scratching an itchy face. (claws image by Undy from

Kidney Disease

Methimazole can reveal hidden kidney disease in cats with hyperthyroidism. Hyperthyroidism causes high blood pressure and heart disease, increasing blood flow through the kidneys, forcing them to function more efficiently. When the feline begins methimazole treatment, Mar Vista Vet states the blood flow returns to normal and any disease within the kidneys is reported accurately through blood work.

Side effects of tapazole in felines
Hyperthyroidism increases blood pressure, masking feline kidney disease. (blood pressure manometer studio isolated image by dinostock from

Liver Failure

Although rare (fewer than 2 per cent of cats treated), liver failure is a serious side effect of methimazole. Liver failure is caused by a toxic reaction to the medication and, according to Mar Vista, toxicity levels generally correct themselves once methimazole treatment is stopped. In serious cases, Doctors Foster and Smith reports the cat may become jaundiced (yellowing of the eyes, skin and gums).

Side effects of tapazole in felines
Yellow in the whites of the cat's eyes indicates jaundice. (cat eyes image by Gareau Enterprises from

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