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Methimazole (Tapazole®)

By: Dr. Dawn Ruben

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  • Hyperthyroidism (overactive thyroid gland) is a very common problem in older cats. The condition leads to weight loss, abnormal appetite and behavior and heart problems. Treatment can include surgery, radioactive iodine or drugs that reduce thyroid function and bring the levels of thyroid hormone back toward normal.
  • Methimazole belongs to a class of drugs that is known as anti-thyroid agents.
  • Iodine is an important part of thyroid function, and methimazole interferes with iodine so that thyroid hormone cannot be effectively formed.
  • Methimazole is used in cats only.
  • Methimazole is a prescription drug and can only be obtained from a veterinarian or by prescription from a veterinarian.
  • This drug is not approved for use in animals by the Food and Drug Administration but it is prescribed legally by veterinarians as an extra-label drug.

    Brand Names or Other Names

  • This drug is registered for use in humans only.
  • Human formulations: Tapazole® (Lilly)
  • Veterinary formulations: None

    Uses of Methimazole

  • Methimazole is used to medically treat overactive thyroid glands in cats, a condition referred to as hyperthyroidism.

    Precautions and Side Effects

  • While generally safe and effective when prescribed by a veterinarian, methimazole can cause side effects in some animals.
  • Methimazole should not be used in animals with known hypersensitivity or allergy to the drug.
  • Methimazole should be used with extreme caution in animals with liver impairment, immune-system disease or blood disorders.
  • Adverse effects occur with some frequency. The most common effects are lack of appetite, vomiting, lethargy, bleeding disorders, liver disease and various blood-cell disorders. In cats taking this drug long term, periodic blood tests are indicated to monitor for toxic effects on blood cells and the liver.
  • Due to the potential for adverse effects, methimazole is commonly used as a temporary treatment for overactive thyroid until surgical removal of the thyroid gland or radiation treatment can be done.

    How Methimazole Is Supplied

  • Methimazole is available in 5 mg and 10 mg tablets.

    Dosing Information

  • Medication should never be administered without first consulting your veterinarian.
  • Initially, methimazole is given at a dose of 5 mg per cat, three times daily.
  • If there is no improvement in thyroid hormone levels, the dose may be slowly increased.
  • Once stable, the dose can be reduced in some - but not all - cats.
  • The duration of administration depends on the condition being treated, response to the medication and the development of any adverse effects. Be certain to complete the prescription unless specifically directed by your veterinarian. Even if your pet feels better, the entire treatment plan should be completed to prevent relapse or prevent the development of resistance.

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