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Developed in collaboration with Andrea Johnson, DVM | Co-Founder | PetVet365
Last reviewed: 9/20/2023
Methimazole is a medication commonly prescribed by veterinarians for the long-term medical management of hyperthyroidism in cats (Feline Hyperthyroidism).
Your veterinarian may prescribe methimazole under the brand names Tapazole® or Felimazole®. It can be given as an oral tablet, oral liquid, oral suspension, or applied topically as a transdermal gel, usually in the cat's ear.
Methimazole is an antithyroid medication that’s used to treat hyperthyroidism in cats. Hyperthyroidism is a condition where your cat’s thyroid gland is overactive. It is most common in older cats. If left untreated, it can cause serious health problems such as heart disease, blindness, weight loss and vomiting/diarrhea.
Methimazole is given long-term to manage thyroid hormone levels and can also be used to normalize thyroid levels before thyroid surgery. Treatment with Methimazole is a form of medical management; it does not cure hyperthyroidism.
Methimazole works by blocking the production of thyroxine (T4), a hormone produced by the thyroid gland, which helps to lower thyroid hormone levels in the blood.
Hyperthyroidism occurs when the thyroid gland produces excessive amounts of thyroid hormones. Methimazole inhibits the body's production of thyroid hormones by interfering with some of the metabolic steps. It has no effect on pre-existing circulating or stored thyroid hormones and it has no effect on supplemented thyroid hormones.
It can take 2-4 weeks before you notice a change in your cat’s symptoms.
Adverse side-effects due to methimazole use usually occur within the first three months of therapy.
Use caution when handling Methimazole and your cat’s bodily fluids or stool. Exposure can be harmful to humans. Pregnant and nursing women, or women who may become pregnant should wear gloves when handling the medication, cat litter, or body fluids. Anyone applying the transdermal gel should wear gloves during application.
If you are exposed to Methimazole, you may experience side effects. These can include vomiting, headache, and abdominal pain. Let your healthcare provider know right away if this happens.
Veterinarians prescribe a specific dosage of methimazole based on your pet’s weight and condition. It is given orally as a tablet, oral suspension, or applied as a transdermal gel. The most commonly used dosing strengths of methimazole are 2.5mg, 5mg, and 10mg. Be sure to follow your veterinarian’s dosing instructions carefully.
Methimazole is a very bitter medication. If your veterinarian determines your pet has special needs that are not satisfied by the commercially available methimazole medication, they may prescribe compounded methimazole that is both the appropriate dose and strength for your cat from a compounding pharmacy. In some instances, a compounded formulation of transdermal preparation is prescribed.
Wedgewood Pharmacy specializes in compounded products and provides medication options that help ensure accurate dosing, especially for uncooperative cats.
Wedgewood Pharmacy provides medication options that help ensure accurate dosing, especially for hard to medicate pets. Click below for a complete list of Wedgewood’s dosing forms and strengths.
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Wedgewood Pharmacy provides medication options that help ensure accurate dosing, especially for hard to medicate pets. Click below for a complete list of Wedgewood’s methimazole dosing forms and strengths.
If you miss giving your pet a dose, give the next dose as soon as you remember or, if it is close to the next scheduled dose, return to the regular schedule. Do not double dose to catch up. If you are not sure what to do, call your veterinarian and follow their directions.
Methimazole metabolizes quickly and overdose effects should not last more than 24 hours but may last longer in pets with liver or kidney disease. Symptoms of a methimazole overdose are generally stronger incidences of the side effects listed above.
If you suspect your pet or another animal has accidentally overdosed or has eaten this medication inadvertently, immediately contact your veterinarian or the A.S.P.C.A.’s Animal Poison Control Center at 888-426-4435. Remember to take your prescription container with you when you take your pet for treatment.
If you or someone else has ingested this medication, call the National Capital Poison Center at 800-222-1222.
Wedgewood Pharmacy’s methimazole preparations start at $0.13 per dose. Your veterinarian will prescribe a specific dosage based on the pet’s weight, condition, and other factors.
Compounded medicines are prepared for the exact strength your veterinarian prescribes. The price of the medication will depend on the dosage and the medication form, with certain dosage forms and higher strengths generally being more expensive.
In addition, the cost of a medication will depend upon the price of the other active pharmaceutical ingredients and may increase the cost of the finished drug.
Looking for Methimazole?
We can let your veterinarian know that you are interested in our compounded Methimazole.
There are several other treatment options for hyperthyroidism in cats. Which one is best for your cat depends on the cause of its hyperthyroidism, its age, and its other health conditions.
No. Methimazole doesn’t cure the cause of feline hyperthyroidism, but it does help keep thyroid hormone levels under control. It is likely that your cat will be on methimazole for the rest of its life.
Methimazole is an antithyroid medication that's used to treat hyperthyroidism. It's given long term to manage thyroid hormone levels in cats. It can also be used to normalize levels before thyroid surgery. Methimazole works by blocking thyroid hormones from being made.
After diagnosis, a cat's life expectancy depends on many factors. One study found that cats who were treated with methimazole followed by radioactive iodine (I-131) typically lived for another 5.3 years.
It is possible to be exposed to methimazole while handling the medication or through your cat’s vomit, urine, or stool. You should wear gloves when cleaning up vomit or emptying the litter box and when you are administering the medication, particularly in topical gel form. Don’t break or crush tablets.
If you’re exposed to methimazole, you may experience vomiting, headache, and abdominal pain. It can also cause harm to babies, so pregnant or nursing mothers should use extra caution.
Contact your healthcare provider right away if you think you are having adverse effects from methimazole.
Your cat should be monitored closely for adverse effects such as tiredness, vomiting, loss of appetite, yellowing of the skin or eyes, or itchiness. If these are seen, discontinue the medication and contact your veterinarian.
Your veterinarian will most likely check your cat’s blood before starting methimazole to establish a baseline of thyroid function, and then check again every 2-3 weeks for the first 3 months to track thyroid hormone levels. Once dosing is stabilized, thyroid levels should be checked every 3-6 months.
If your cat has liver disease, your veterinarian may monitor liver enzymes to ensure it is not affecting renal function.
Methimazole should not be used in cats that are pregnant or nursing and in those with pre-existing liver or hematological diseases such as anemia, platelet disorders, and in cats with autoimmune diseases.
It depends on the cat’s metabolism. Methimazole typically takes between 2-4 weeks to take effect. During this time, it is important to observe and record your cat’s symptoms and behavior so you can report them during your next visit. Your veterinarian may need to fine tune the dosage up or down depending on your cat’s response to methimazole.
This article is meant to provide general and not medical advice. We strongly recommend that a veterinarian be consulted for the specific medical needs of your animal.
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Methimazole for Veterinary Use
Andrea Johnson, DVM, is co-founder of PetVet365, a franchise company that creates new veterinary practices around entrepreneur owners determined to reinvent the animal healthcare profession and to offer the highest quality care for pets and their families.
She began her career as an associate veterinarian with a practice in Kentucky and eventually became owner and chief medical officer for 15 Banfield Pet Hospital franchises in Kentucky, Ohio, and Indiana, with 75 veterinarians on her team. She was a veterinary consultant for LegacyVet and a self-employed consultant prior to co-founding PetVet365.
She holds a BS degree in biology from Marshall University, an MS degree in Biology and Biological Sciences from Marshall University, and a DVM degree from the University of Georgia College of Veterinary Medicine.
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This content is intended for counseling purposes only. This content is informational/educational and is not intended to treat or diagnose any disease or patient. No claims are made as to the safety or efficacy of mentioned preparations. The compounded medications featured in this content have been prescribed and/or administered by prescribers who work with Wedgewood Pharmacy. You are encouraged to speak with your prescriber as to the appropriate use of any medication. Wedgewood Pharmacy’s compounded veterinary preparations are not intended for use in food and food-producing animals. All product and company names are trademarks™ or registered® trademarks of their respective holders. Use of them does not imply any affiliation with or endorsement by them.