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Methimazole for Cats

By Barbara Forney, VMD

Last reviewed: 7/13/2022

Commonly prescribed for: Medical Management of Hyperthyroidism

Species: Cats

Therapeutic Class: Antithyroid Agent

General Drug Information and Indications

Methimazole is a human drug that is used for the medical management of hyperthyroidism in cats. It currently is the drug of choice for the treatment of over-active thyroid and has largely replaced propylthiouracil for this purpose due to the lower incidence of side effects. Treatment with methimazole is a form of medical management; it does not cure the condition.

Cats that are starting treatment should be monitored closely for the first three months. Blood counts and thyroid hormone levels should be measured every two to three weeks. Other blood tests for liver function and immune function may be performed as needed. The cat's natural thyroid hormone levels will be reduced in one to three weeks. After your cat is stabilized on methimazole, blood tests for thyroid hormone levels usually are performed every three to six months.

Methimazole also is being used experimentally to protect the kidneys in dogs that are receiving cisplatin chemotherapy.

If your veterinarian determines your pet has special needs that are not satisfied by the commercially available methimazole medication, he or she may prescribe compounded methimazole that is both the appropriate size and strength for your cat from a compounding pharmacy. In some instances a compounded transdermal preparation is prescribed.

How to Give this Medication

Give this medication to your pet exactly as your veterinarian prescribes. If you miss giving your pet a dose of methimazole, 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.

Methimazole is a very bitter medication. Different formulations of the medication are available and giving the medication with food may improve may improve palatability.

Methimazole is available in a transdermal cream or gel, which can be rubbed on the inside of your cat's ear. If you use the transdermal preparation, be sure to wear gloves when applying the medication.

Wash your hands after giving your pet this medication.

Side Effects

Be sure to discuss any side-effects with your veterinarian immediately. Adverse side effects usually occur within the first three months of therapy.

The most-common side effect is upset of the digestive tract. Up to 20% of cats may experience nausea, vomiting, loss of appetite and just not feel well. Usually this is self-limiting and most cats stop vomiting and begin to feel better without any change to the medication. A small number of cats will need to have their dose reduced.

There is a lower incidence of digestive upset in cats that are receiving the transdermal gel, but it may take longer before the full effect of the transdermal treatment can be seen.

Within the first few weeks of treatment, a small number of cats self mutilate their face and neck through scratching. These animals will probably need to discontinue treatment.

Temporary changes in blood counts are seen in about 15% of cats. These usually occur within the first two months of therapy. A very small number of cats will develop very serious changes in their bone marrow, blood counts or liver problems. Approximately 50% of animals receiving methimazole for more than six months develop other blood abnormalities (positive ANA). These cats may require a dose reduction.

Very rarely, acquired myasthenia gravis (a disease that weakens muscles) may occur.


Keep this and all drugs out of reach of children. Methimazole is a prescription drug and should be used according to your veterinarian's directions, and given only to the animal for which it was prescribed. Do not give this medication to a person.

Methimazole should be avoided or used with extra monitoring in cats with liver disease, autoimmune disease or pre-existing blood abnormalities.

Individual response to methimazole may vary. Regular monitoring of thyroid hormone levels is necessary in order to avoid drug-induced hypothyroidism.

Kittens born to cats receiving this drug may be born with low thyroid levels. Your veterinarian may recommend a milk replacement for your kitten.

Drug Interactions

Be sure to review with your veterinarian any medications or supplements your pet may be receiving.

Bupropion may increase the possibility of liver toxicity. If bupropion is used with methimazole, increased monitoring of liver function may be advisable. Methimazole may decrease the efficacy of digoxin. Methimazole may decrease the anticoagulant actions of warfarin.


If you suspect your pet or another animal was accidentally overdosed or has eaten this medication inadvertently, contact your veterinarian or the A.S.P.C.A.'s Animal Poison Control Center at 888.426.4435. Always bring the prescription container with you when you take your pet for treatment.

If you or someone else has accidentally ingested this medication call the National Capital Poison Center at 800.222.1222.


Different strengths or dosage forms of methimazole may have different storage requirements. Read the labeling or ask your pharmacist for the storage requirements of the prescription you receive.

More Information About This Medication

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.
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