Methimazole

Medically reviewed by Alex Brewer, PharmD, MBA on May 30, 2023
Written by Jennifer Mitri Williamson, PharmD

This article describes methimazole’s uses, side effects, and more. Below you’ll also find some coupon options to help save on the cost of methimazole.

Methimazole may cause mild or serious side effects (also known as adverse effects). More common mild side effects of methimazole and its serious side effects are listed below. This article doesn’t include all possible side effects of the drug. Side effects can vary based on your age, overall health, and any other medications you take.

To learn more about methimazole’s side effects, ask your doctor or pharmacist. You can also read the prescribing information* for methimazole.

* To view methimazole’s prescribing information, see the “Article resources” section below.

Mild side effects

More common mild side effects reported with methimazole are listed below.

This article doesn’t include all possible mild side effects of the drug. To learn more about methimazole’s side effects, see the prescribing information* for methimazole.

With many drugs, mild side effects may go away within a few days or a couple of weeks. If the side effects are bothersome, tell your doctor or pharmacist. They may be able to suggest ways to manage them.

Methimazole oral tablet’s mild side effects include:

• skin rash
• nausea, vomiting, or both
• joint pain
• itchiness
• mild allergic reaction†

* To view methimazole’s prescribing information, see the “Article resources” section below.
† For details about this side effect, see the “Interactions and warnings for methimazole” section below.

Serious side effects

Serious side effects of methimazole are listed below.

This article doesn’t include all possible serious side effects of the drug. To learn more about methimazole’s side effects, see the prescribing information* for methimazole.

With many drugs, serious side effects are possible but not common. If you have serious side effects from this drug, call your doctor right away. If you’re having severe symptoms or a medical emergency, call 911 or a local emergency number.

In general, methimazole’s serious side effects include:

• decreased blood cell levels, including low levels of platelets and red blood cells
• liver problems, including hepatitis (inflammation of the liver)
• kidney problems
• inflammation (swelling) of the blood vessels, such as vasculitis or periarteritis
• low level of prothrombin (a protein involved in blood clotting), which may cause bleeding that lasts longer than usual
• insulin autoimmune syndrome (an immune system reaction to the insulin your body produces, leading to low blood sugar)
• lupus-like syndrome (an immune system reaction that causes similar symptoms to lupus)
• drug fever (a fever that occurs when you take certain medications)
• low thyroid activity
• severe allergic reaction†

* To view methimazole’s prescribing information, see the “Article resources” section below.
† For details about this side effect, see the “Interactions and warnings for methimazole” section below.

Below you’ll find answers to a few commonly asked questions about methimazole.

Can methimazole be used in cats?

Yes, but it’s not the same as methimazole that’s used in humans. The human form of methimazole is different from the form used in cats.

Methimazole for cats is available as generic methimazole. It’s also available as the brand-name drug Felimazole.

Talk with your veterinarian to learn more about treatments for overactive thyroid in cats, including the forms of methimazole approved for this use.

How does methimazole compare with carbimazole or thiamazole?

Like methimazole, carbimazole is used to treat an overactive thyroid. However, carbimazole isn’t approved for use in the United States. It’s approved for use in other countries, including Australia and many European countries. 

Methimazole is also called thiamazole. So they are the same medication and have the same side effects and dosages.

There are other options in addition to methimazole for overactive thyroid. They include propylthiouracil (PTU), iodine therapy, or surgery.

If you have questions about the best treatment option for your condition, talk with your doctor or pharmacist. They can help determine the best treatment option for you.

Will I experience certain side effects when stopping methimazole?

It’s possible for you to experience side effects when stopping methimazole. Methimazole is used to treat an overactive thyroid. So, the drug works by slowing down the amount of thyroid hormone that your body makes.

If you stop taking methimazole, your thyroid may become overactive again. This means that your body may begin to produce too much thyroid hormone again. So if you stop taking methimazole, you may experience symptoms of overactive thyroid. Symptoms may include:

• weight loss
• anxiety
• panic attacks
• hair loss
• fast heart rate

If you’d like to stop taking methimazole, talk with your doctor. They can recommend how and when to stop treatment with methimazole. Also, they may recommend a different treatment option for your condition.

What is the best time of day to take methimazole?

Methimazole is usually taken three times per day, separated by about 8 hours per dose. For example, if you take your first dose at 7 a.m., you will take your second dose at 3 p.m. and your last dose at 11 p.m.

Before you start taking methimazole, your doctor or pharmacist will discuss the best times to take your medication.

Is weight gain a side effect of methimazole?

No, weight gain wasn’t a side effect that people taking methimazole reported in studies. However, having an overactive thyroid may cause you to lose weight. So if you have treatment for your overactive thyroid, you may gain some weight.

If you notice weight gain that’s bothersome, talk with your doctor. They can help recommend the best ways to manage your weight.

How does methimazole work? And how long does it take to start working?

Methimazole works to treat an overactive thyroid in certain adults and children. It works by slowing down the amount of thyroid hormones that your body makes.

The drug begins working as soon as you take your first dose.

If you have more questions about how methimazole works or when it should start working, talk with your doctor or pharmacist.

The dosage of methimazole your doctor prescribes may vary based on your condition and certain other factors. Talk with your doctor about the dosage you should follow.

Taking methimazole

Methimazole oral tablet is swallowed. The manufacturer of this medication doesn’t specify whether the drug should be taken with or without food. If you have questions about how to take methimazole, talk with your doctor or pharmacist.

Your doctor will talk with you about how to take methimazole. They’ll explain how much to take and how often. Always follow your doctor’s recommendation.

See the “Common questions about methimazole” section for information on missed doses and the best time to take this drug.

Overdose of methimazole

You should not take more methimazole than your doctor prescribes. For some drugs, doing so may lead to serious side effects or overdose.

If you think you’ve taken too much methimazole, call your doctor or pharmacist right away. Or you could call 800-222-1222 to speak with someone at America’s Poison Centers. You can also use its online resource. If you have concerning symptoms, call 911 or a local emergency number immediately. You can also go to the closest emergency room.

The Food and Drug Administration (FDA) approves prescription drugs such as methimazole for certain conditions. Approved uses for methimazole are described below.

Use for hyperthyroidism

Doctors may prescribe methimazole oral tablet for treating hyperthyroidism (overactive thyroid). It’s used for this purpose in certain adults and children. Methimazole may be a short-term or a long-term treatment option.

Specifically, methimazole may be used in people with Graves’ disease with overactive thyroid or goiter who cannot have surgery or iodine therapy. (Graves’ disease is an autoimmune condition that causes an overactive thyroid. Goiter is a condition that increases the size of your thyroid gland.) In this case, the drug may be used as a long-term treatment option.

Methimazole may also be used in people with overactive thyroid that are getting ready for thyroid surgery or iodine therapy. In this case, the drug may be used as a short-term treatment option.

With an overactive thyroid, your thyroid gland makes too much thyroid hormone. This may cause symptoms such as:

• weight loss
• fast heart rate
• anxiety
• sweating
• goiter

Below, you’ll find information about methimazole’s possible interactions and warnings.

Interactions of methimazole

Certain things may affect how a drug works. These include getting vaccines, consuming alcohol or certain foods, or taking the drug with other medications. This effect is called a drug interaction.

Before you take methimazole, ask your doctor to check for possible interactions. Be sure to tell them about any of the following you take or use:

• prescription drugs
• over-the-counter medications
• vitamins, herbs, or supplements

To learn about drug-condition interactions, see the “Warnings for methimazole” section below.

Warnings for methimazole

Some people should not take methimazole, and others should take it cautiously.

Methimazole could cause harm to people with certain health conditions. This effect is called a drug-condition interaction. Other factors can also affect whether methimazole is a safe option for you. These warnings may differ based on the form of methimazole you take.

Ask your doctor about specific warnings for methimazole, and be sure to tell your doctor about your:

• current health, including any allergies to medications
• past health conditions or surgeries

Allergic reaction

Methimazole can cause an allergic reaction in some people. Severe allergic reactions are rare but possible.

If you’ve had an allergic reaction to methimazole or any of its ingredients, your doctor will likely not prescribe methimazole. They can tell you about medications that are safer options for you.

A mild allergic reaction may cause the following symptoms:

• skin rash
• itchiness
• flushing (temporary warmth, redness, or deepening of skin color)

A severe allergic reaction may cause the following symptoms:

• swelling under your skin, usually in your hands, feet, eyelids, or lips
• swelling of your mouth, throat, or tongue, which can cause breathing problems

If you have an allergic reaction to methimazole, call your doctor right away. If you have severe symptoms, call 911 or a local emergency number.

Pregnancy or breastfeeding and methimazole

Information about methimazole and pregnancy and breastfeeding is described below.

Methimazole and pregnancy

There are risks to taking methimazole during pregnancy. But there are also risks with having an untreated thyroid condition while pregnant. (Keep in mind that methimazole is used for treating an overactive thyroid.)

If you’re planning a pregnancy or can become pregnant, talk with your doctor about overactive thyroid treatment options during pregnancy. Together, you can discuss the risks and benefits of your treatment options and decide whether you should take methimazole during pregnancy.

Methimazole and breastfeeding

It’s not known for sure whether methimazole should be taken while breastfeeding. The drug passes into breast milk, but it doesn’t seem to cause problems in children who are breastfed. If you’re currently breastfeeding or planning to do so, talk with your doctor before taking this medication.

You may be able to save money on your prescription for methimazole by using our Perks discount coupons. They can be found at the end of this article.

If you have questions about how to pay for methimazole, talk with your doctor or pharmacist.

Note: Optum Perks coupons cannot be used with insurance copays or benefits.

This article describes methimazole’s uses and dosage, ways to save on cost, and more. Let your doctor know if you have questions about methimazole or would like more details about it.

Here’s a list of some possible questions you may want to ask your doctor:

• Is methimazole available as a brand-name drug?
• What other treatment options are available if I can’t afford methimazole?
• If I experience side effects from this medication, will you decrease my dose?

• Food and Drug Administration. (2023). Orange Book: Approved drug products with therapeutic equivalence evaluations. https://www.accessdata.fda.gov/scripts/cder/ob/index.cfm
• Hyperthyroidism in cats—There’s an FDA-approved drug to treat it. (2019). https://www.fda.gov/animal-veterinary/animal-health-literacy/hyperthyroidism-cats-theres-fda-approved-drug-treat-it
• Methimazole tablets, USP. (2023). https://nctr-crs.fda.gov/fdalabel/services/spl/set-ids/c511344b-cc7f-499b-bb21-5b2312ff311e/spl-doc

Disclaimer: Optum Perks has made every effort to make certain that all information is factually correct, comprehensive, and up to date. However, this article should not be used as a substitute for the knowledge and expertise of a licensed healthcare professional. You should always consult your doctor or another healthcare professional before taking any medication. The drug information contained herein is subject to change and is not intended to cover all possible uses, directions, precautions, warnings, drug interactions, allergic reactions, or adverse effects. The absence of warnings or other information for a given drug does not indicate that the drug or drug combination is safe, effective, or appropriate for all patients or all specific uses.