What is methimazole?
Methimazole (brand names Tapazole®, Felimazole®, Felanorm®) 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”, which means the drug is being used in a way that has not been reviewed or approved by the U.S. Food and Drug Administration (FDA) or Health Canada. In these instances, follow your veterinarian’s directions and cautions carefully, as they 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 ear flap) in the form of a transdermal gel.
- The oral form can be given with food or an empty stomach.
- 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.
- 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.
- Wear gloves when applying the transdermal gel.
- When applying the transdermal gel, ensure the dose is rubbed in until fully absorbed into the skin. Prevent other animals and children from touching the application site for two hours after the drug is administered.
Methimazole can take days to weeks before effects are noted, and sometimes the effects are not obvious. This medication must be given for life, as it manages hyperthyroidism; it does not provide a cure.
What if I miss giving my pet the medication?
If you miss a dose, give it as soon as you remember, and then continue with the regular dosing schedule. However, if it is almost time for the next dose, skip the missed dose and resume the regular dosing schedule. Do not give the pet two doses at once.
Are there any potential side effects?
Methimazole commonly causes the following side effects, usually within the first three months of treatment:
- vomiting
- anorexia
- tiredness
Less commonly, Methimazole may cause the following side effects, which are generally temporary and resolve with time:
- gastrointestinal effects
- mild changes in the pet’s blood cell counts
Rarely, the following side effects may occur:
- itchiness
- facial scratches
- severe changes in blood cell counts
- liver disease
- immune- mediated disorders
- myasthenia gravis (a neuromuscular disease)
This short-acting medication should last less 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 allergic to carbimazole or polyethylene glycol
- pets that are breeding, pregnant, or nursing
- cats with autoimmune disease, liver disease, kidney disease, blood and clotting diseases (or used with extreme caution and significant monitoring)
If household cats groom each other (i.e., allogrooming), the transdermal formulation may not be an appropriate option.
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
- warfarin
Be sure to tell your veterinarian about any medications (including vitamins, supplements, or herbal therapies) that your pet is taking.
Is any monitoring needed 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?
- Store methimazole tablets in a dry, dark place, at room temperature.
- Protect from light and moisture.
- Store compounded products 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.
