What Is Methimazole?
Methimazole is a medicine that affects the function of your thyroid gland, which is the butterfly-shaped gland in the front of your neck. Your thyroid produces the hormones triiodothyronine (T3) and thyroxine (T4), which help regulate your metabolism, mood, digestion, and muscle control.
A condition called Graves’ disease occurs when the thyroid gland is overactive and produces excess thyroid hormones, also called hyperthyroidism. Methimazole is a medicine that comes as a pill taken by mouth to treat hyperthyroidism. It lowers the amount of T3 and T4 that your thyroid gland produces.
The brand name Tapazole is discontinued, but methimazole is available generically. Methimazole is a prescription product, so you can’t purchase it over the counter. You’ll receive a prescription from your healthcare provider and get the medication from your pharmacy.
Generic Name: Methimazole
Brand Name: Tapazole (discontinued)
Drug Availability: Prescription
Therapeutic Classification: Antithyroid agent
Available Generically: Yes
Controlled Substance: N/A
Administration Route: Oral
Active Ingredient: Methimazole
Dosage Form: Tablet
What Is Methimazole Used For?
The Food and Drug Administration (FDA) approved methimazole to treat:
- Graves’ disease with hyperthyroidism or a toxic multinodular goiter (where your thyroid gland enlarges and produces too much T3 and T4) is not an option for surgery or iodine therapy. Iodine plays an important role in thyroid function and disorders as the gland relies on it to produce hormones
- Symptoms of hyperthyroidism before having thyroid surgery or iodine therapy
How to Take Methimazole
Take methimazole exactly as your healthcare provider prescribes it, between one and three times a day. You may wish to set a phone reminder or make taking methimazole part of your daily routine, like taking it when you wake up, at the same time every afternoon, and at bedtime.
You can take this medication with or without food, but try to stay consistent. If you take it with food, always take it with food. If you take it on an empty stomach, always take it on an empty stomach. This will help you get consistent effects from the medicine.
Store methimazole at room temperature (between 59 degrees F to 86 degrees F) in the original container with the lid on, out of reach of children and pets. Avoid storing your pill bottle in an area with a lot of heat and moisture like the bathroom.
If you’re traveling by plane, you’ll want to keep this medication in your carry-on luggage so that you aren’t separated from it if your checked baggage goes missing. If you’re traveling by car, take care not to leave your pill bottle in especially hot or cold temperatures for long periods, like overnight in the car.
How Long Does Methimazole Take to Work?
It will most likely be about three weeks, and sometimes closer to six or eight weeks before you start noticing symptom improvement from taking methimazole. This is because the medicine only blocks the formation of new thyroid hormones, it doesn’t remove the hormones your thyroid has already produced that are causing your current symptoms.
For this reason, it’s important not to miss doses of methimazole. If you frequently miss doses, your thyroid may start producing hormones again, causing symptoms.
What Are the Side Effects of Methimazole?
This is not a complete list of side effects and others may occur. A healthcare provider can advise you on side effects. If you experience other effects, contact your healthcare provider. You may report side effects to the FDA at fda.gov/medwatch or 1-800-FDA-1088.
Common Side Effects
Some more common side effects that you may experience while taking methimazole include the following. If any of these side effects do not go away or feel severe, contact your healthcare provider on how to proceed:
- Urticaria, a red skin rash accompanied by itching
- Nausea or vomiting
- Upset stomach
- Paresthesias, which are painless, tingling, or “skin-crawling” sensations that can occur in the arms, legs, hands, or face
- Loss of taste
- Myalgia, or muscle pain
- Pruritus, or itching
Severe Side Effects
These are some more serious side effects that you and your healthcare provider should watch out for:
- Acute pancreatitis is when your pancreas can become inflamed over a short period. Sometimes this condition can be serious. Make sure to report to your healthcare provider any symptoms like upper abdominal pain or abdominal pain that radiates to your back, fever, fast heartbeat, nausea, or vomiting.
- Vasculitis, which is inflammation and/or narrowing of the blood vessels. Symptoms may include fatigue, fever, weight loss, and muscle or joint pains.
- Blood problems like leukopenia (low white blood cells), thrombocytopenia (low platelets, which can make bruising and bleeding easier), or aplastic anemia (when blood cells are damaged). If you experience symptoms like a fever or sore throat, let your healthcare provider know as soon as possible.
Call your healthcare provider right away if you feel like you are experiencing serious side effects. Call 911 if your symptoms feel life-threatening or if you think you’re having a medical emergency.
Long-Term Side Effects
Methimazole is shown to be safe with long-term use in treating Graves’ disease, and no separate side effects apart from those listed above are shown to be more likely with a longer time spent taking methimazole.
Report Side Effects
Methimazole may cause other side effects. Call your healthcare provider if you have any unusual problems while taking this medication.
If you experience a serious side effect, you or your healthcare provider may send a report to the FDA’s MedWatch Adverse Event Reporting Program or by phone (800-332-1088).
Dosage: How Much Methimazole Should I Take?
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The dose of this medicine will be different for different patients. Follow your doctor’s orders or the directions on the label. The following information includes only the average doses of this medicine. If your dose is different, do not change it unless your doctor tells you to do so.
The amount of medicine that you take depends on the strength of the medicine. Also, the number of doses you take each day, the time allowed between doses, and the length of time you take the medicine depend on the medical problem for which you are using the medicine.
For oral dosage form (tablets):
- Adults—At first, 15 to 60 milligrams (mg) per day, given in 3 divided doses about every 8 hours. The maintenance dose is 5 to 15 mg per day.
- Children—Dose is based on body weight and must be determined by your doctor. The starting dose is 0.4 mg per kilogram (kg) of body weight per day, divided in 3 equal doses and given about every 8 hours. The maintenance dose is 0.2 mg per kg of body weight per day.
- For hyperthyroidism:
In some cases, you may need to modify or change your treatment regimen with methimazole.
Methimazole should not be taken while pregnant. Harm can occur to the fetus like fetal goiter, cretinism, and hypothyroidism (low thyroid hormones) when a pregnant person takes methimazole. Sometimes, thyroid function decreases during pregnancy, so you may need less medicine to treat this anyway.
However, balanced maternal thyroid hormone levels are essential for a healthy pregnancy. If methimazole must be used, it should be at the lowest dose possible and only after an individual risk versus benefit evaluation with your healthcare provider.
This medication does get excreted into human breast milk, and it should not be used in people who are nursing.
Compared to other thyroid lowering medicines, methimazole is the preferred drug in children with hyperthyroidism.
If you miss a dose, you can take it as soon as you remember, unless it is close to being time for your next dose. For example, if you take methimazole at 6 a.m., 2 p.m., and 10 p.m., and you remember at 7 p.m. that you forgot your 2 p.m. dose, skip that dose and take your next one as scheduled at 10 p.m. Do not double up doses to make up for missed ones.
It’s important not to miss doses of methimazole, since the medicine works to block the formation of new thyroid hormones. It does not affect the hormones your thyroid has already made that are causing your symptoms. If you do frequently miss doses, your thyroid may start producing hormones again, causing symptoms.
Overdose: What Happens If I Take Too Much Methimazole?
If you take too much methimazole, you may experience symptoms like:
- Nausea and vomiting
- Upset stomach
- Joint pain
It’s possible for more serious problems to arise as well, such as issues with your blood or liver. If your situation seems severe, call 911 or go to the emergency room immediately.
What Happens If I Overdose on Methimazole?
If you think you or someone else may have overdosed on methimazole, call a healthcare provider or the Poison Control Center (800-222-1222).
If someone collapses or experiences signs of an overdose after taking methimazole, call 911 immediately.
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It is very important that your doctor check your or your child’s progress at regular visits to make sure that the medicine is working properly. Blood and urine tests may be needed to check for unwanted effects.
Using this medicine during the first three months of your pregnancy can harm your unborn baby. Use an effective form of birth control to keep from getting pregnant. If you think you have become pregnant while using the medicine, tell your doctor right away.
In very rare situations, methimazole can temporarily lower the number of white blood cells in your blood, which increases the chance of getting an infection. It can also lower the number of platelets, which are necessary for proper blood clotting. If this occurs, there are certain precautions you can take, especially when your blood count is low, to reduce the risk of infection or bleeding:
- If you can, avoid people with infections. Check with your doctor immediately if you think you are getting an infection or if you get a fever or chills, cough or hoarseness, lower back or side pain, or painful or difficult urination.
- Check with your doctor immediately if you notice any unusual bleeding or bruising, black, tarry stools, blood in the urine or stools, or pinpoint red spots on your skin.
- Be careful when using a regular toothbrush, dental floss, or toothpick. Your medical doctor, dentist, or nurse may recommend other ways to clean your teeth and gums. Check with your medical doctor before having any dental work done.
- Do not touch your eyes or the inside of your nose unless you have just washed your hands and have not touched anything else in the meantime.
- Be careful not to cut yourself when you are using sharp objects such as a safety razor or fingernail or toenail cutters.
- Avoid contact sports or other situations where bruising or injury could occur.
Check with your doctor right away if you have pain or tenderness in the upper stomach, pale stools, dark urine, loss of appetite, nausea, vomiting, or yellow eyes or skin. These could be symptoms of a serious liver problem.
This medicine may cause hypothyroidism (low levels of thyroid hormone in the blood). Check with your doctor right away if you have constipation, a depressed mood, dry skin and hair, feeling cold, hair loss hoarseness or husky voice, muscle cramps and stiffness, slowed heartbeat, weight gain, or unusual tiredness or weakness.
Make sure any doctor or dentist who treats you knows that you are using this medicine.
Do not take other medicines unless they have been discussed with your doctor. This includes prescription or nonprescription (over-the-counter [OTC]) medicines and herbal or vitamin supplements.
What Are Reasons I Shouldn’t Take Methimazole?
Methimazole may not be a good option for you if you:
- Are allergic to methimazole or any of the ingredients in methimazole
- Are pregnant or breastfeeding, as this medication may cause harm to your fetus and is also transferred into your breast milk
- Currently have or in the past have had problems with your pancreas after taking methimazole
- Have a low white blood cell count
- Have liver problems
- Are intolerant to some sugars, such as lactose (a milk sugar that is a component of methimazole)
- Are taking medications that are known to cause agranulocytosis (decrease in white blood cell count)
What Other Medications Interact With Methimazole?
The following drugs may interact with methimazole:
- Warfarin, a blood thinner: Methimazole can inhibit vitamin K, which can lead to further blood thinning. Since warfarin is already thinning your blood, the dose of warfarin may need to be decreased to prevent bleeding and bruising.
- Beta-blockers like Toprol XL or Lopressor (metoprolol), Tenormin (atenolol), and Brevibloc (esmolol): People with hyperthyroidism may metabolize beta-blockers quicker than those with regular thyroid activity. Since using methimazole aims to achieve regular thyroid activity, you may start to metabolize beta-blockers more slowly and, therefore, need to reduce their dose.
- Digitalis: People with hyperthyroidism metabolize digitalis quicker than people with normal thyroid activity. You may start to metabolize digitalis more slowly as your thyroid activity normalizes and therefore need to reduce the dose.
- Theophylline: Hyperthyroidism can clear theophylline more quickly from your system. When you achieve regular thyroid activity with methimazole, your theophylline dose may need to be decreased since you will metabolize it more slowly with normal thyroid function.
What Medications Are Similar?
Another antithyroid drug available in the United States is propylthiouracil, often abbreviated to PTU. Compared to methimazole, PTU is more likely to cause liver damage and does not reverse hyperthyroidism as quickly.
PTU is not a drug you should take along with methimazole. Ask your pharmacist or a healthcare provider if you have questions.
Frequently Asked Questions
What is methimazole used for?
Methimazole is used to treat hyperthyroidism, also called Graves’ disease. This condition occurs when your thyroid gland produces too much T3 and T4, the two main thyroid hormones. Symptoms include bulging eyes, fast heartbeat, unintentional weight loss, trouble sleeping, and muscle pain.
How does methimazole work?
Methimazole works by preventing your thyroid gland from making more thyroid hormones, resulting in a balanced state and relieving symptoms of hyperthyroidism. It does not affect the hormones your thyroid has already made.
What medicines interact with Tapazole?
Medicines that can interact with methimazole include warfarin, digitalis, theophylline and beta-blockers. Most of these interactions necessitate a dose change since your thyroid is involved in metabolizing drugs. When you change the amount of hormones your thyroid is producing by taking methimazole, the speed at which you metabolize some other drugs changes as well.
How Can I Stay Healthy While Taking Methimazole?
Thyroid disorders are common, with millions of Americans dealing with them. Fortunately, they are treatable conditions. Still, they can be hard to live with and affect your quality of life, especially since mood is one of the things affected by your thyroid hormones. Thyroid disorders are also a lifelong issue, but you can learn to manage them. Maintaining other aspects of your lifestyle, like eating a balanced diet, getting enough sleep and exercise, and prioritizing your mental health by seeking counseling and confiding in loved ones, will help you stay healthy and manage your hyperthyroidism and medications.
While hyperthyroidism is very treatable, be aware of some potential complications that can be dangerous. One such complication is thyroid storm, where your thyroid produces high amounts of T3 and T4 very rapidly. This can cause a high fever, fast heartbeat (up to 200 beats per minute), high blood pressure, shortness of breath, and confusion, among other signs. Thyroid storm is a medical emergency, and you will need to go to the emergency room immediately if you suspect you may be experiencing it.
Verywell Health’s drug information is meant for educational purposes only and is not intended to replace medical advice, diagnosis, or treatment from a healthcare provider. Consult your healthcare provider before taking any new medication(s). IBM Watson Micromedex provides some of the drug content, as indicated on the page.