Methimazole

By Rania Gollakner, BS, DVM

Medications

What is methimazole?

Methimazole (brand names: Tapazole®, Felimazole®) is an antithyroid medication used to treat hyperthyroidism in cats.

Its use in cats and guinea pigs to treat hyperthyroidism is occasionally 'off label' or 'extra label'. Many drugs are commonly prescribed for off label use in veterinary medicine. In these instances, follow your veterinarian’s directions and cautions very carefully as their directions may be significantly different from those on the label.

How is methimazole given?

Methimazole is administered by mouth in the form of an oral tablet or is compounded as an oral liquid. It can also be applied to hairless skin (usually on the inside of the earflap) in the form of a transdermal gel. This medication can take days to weeks before effects are noted, and sometimes effects are not visibly obvious.

The oral form can be given with food or an empty stomach, but if your pet vomits or acts sick after receiving the medication without food, give it with food or a small treat. Follow all directions on the label, especially for compounded medications, and measure the liquid or gel doses carefully. This medication must be given for life, as it manages hyperthyroidism; it does not provide a cure.

Pregnant and nursing women, or women who may become pregnant should wear gloves when handling the medication, cat litter, or body fluids from treated pets. Anyone applying the transdermal gel should wear gloves during administration.

What if I miss giving my pet the medication?

If you miss a dose, give it when you remember, but if it is close to the time for the next dose, skip the dose you missed and give it at the next scheduled time, and return to the regular dosing schedule. Never give your pet two doses at once or give extra doses.

Are there any potential side effects?

The most common side effects include vomiting, anorexia, and tiredness, and usually occur within the first three months of treatment. Other, less common side effects such as gastrointestinal effects and mild changes in your pet’s blood cell counts are usually temporary and resolve with time.

In a small percentage of pets, itchiness, facial scratches, severe changes in blood cell counts, liver disease, and immune-mediated disorders (when the body’s immune system attacks itself) can occur. Rarely, myasthenia gravis, a neuromuscular disease, can occur.

This short-acting medication should not last more than 24 hours, but may last longer in pets with liver or kidney disease.

Are there any risk factors for this medication?

Methimazole should not be used in pets that are allergic to it, or those allergic to carbimazole or polyethylene glycol. Cats with autoimmune disease, liver disease, kidney disease, or blood and clotting diseases should not use this medication, or it should be used with extreme caution and significant monitoring. Breeding, pregnant, or nursing pets should not use this medication.

Are there any drug interactions I should be aware of?

The following medications may interact with methimazole and should be used with caution: benzimidazole antiparasitics, beta-blockers, digoxin, phenobarbital, theophylline, and warfarin.

Be sure to tell your veterinarian about any medications (including vitamins, supplements, or herbal therapies) that your pet is taking.

Is there any monitoring that needs to be done with this medication?

Your pet should be monitored closely for adverse side effects such as tiredness, vomiting, inappetence, yellowing of the skin or eyes, or itchiness. If these are seen, discontinue the medication and contact your veterinarian. Your veterinarian will check your pet’s blood prior to starting the medication and every 2-3 weeks for the first 3 months of therapy to check that the medication is working and also for adverse effects. Once dosing is stabilized, thyroid levels should be checked every 3-6 months.

How do I store methimazole?

Methimazole tablets should be protected from light and moisture and stored in a dry, dark place, at room temperature. Compounded products should be stored according to the compounding pharmacy’s label.

What should I do in case of emergency?

If you suspect an overdose or an adverse reaction to the medication, call your veterinary office immediately. If they are not available, follow their directions in contacting an emergency facility.

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