The Side Effects of Methimazole in Cats

Methimazole is used to treat hyperthyroidism in cats. It is currently the drug of choice, replacing propylthiouracil, which has shown more adverse effects. Methimazole is a human drug that manages, but does not cure, hyperthyroidism. Treatment for thyroid problems is a lifelong need. Side effects appear during the first three months of treatment.

  1. Common Side Effects

    • The common side effects of methimazole are depression, vomiting and loss of appetite. These adverse reactions usually resolve within the first two to three months of treatment. Methimazole is a bitter tasting medicine and should be given with food. This may also help reduce gastrointestinal upset. The thyroid function should be monitored every two weeks to prevent low thyroid function. Methimazole dosages are adjusted as necessary to obtain the optimal thyroid function for the cat. Adjusting the dose may reduce side effects.

    Rare Side Effects

    • Cats that develop hematologic abnormalities should discontinue the medicine. A rare adverse effect occurs in cats that self-injure around their faces and necks. This behavior usually begins as excessive scratching. Cats on methimazole should be monitored for liver, kidney and blood clotting problems. Kittens of mother cats on methimazole may be born with hypothyroidism and require treatment.

    Effects on Older Cats

    • Older cats may show improved kidney or heart function from hyperthyroidism. This must be considered in setting the dosage of methimazole so that benefit is not lost. Cats on methimazole should be monitored for low blood cell counts. Older cats that have compromised immune systems may not tolerate methimazole treatment.