Methimazole belongs to the class of medications called antithyroid medications. It is used to treat hyperthyroidism (overactive thyroid gland). It prevents the thyroid gland from over-producing thyroid hormone, but does not interfere with the actions of thyroid hormone. It may take weeks to months before methimazole has its full effect on the symptoms of overactive thyroid (e.g., palpitations, sweating). During this time period, other medications may be used to control these symptoms.

This medication may be available under multiple brand names and/or in several different forms. Any specific brand name of this medication may not be available in all of the forms or approved for all of the conditions discussed here. As well, some forms of this medication may not be used for all of the conditions discussed here.

Your doctor may have suggested this medication for conditions other than those listed in these drug information articles. If you have not discussed this with your doctor or are not sure why you are taking this medication, speak to your doctor. Do not stop taking this medication without consulting your doctor.

Do not give this medication to anyone else, even if they have the same symptoms as you do. It can be harmful for people to take this medication if their doctor has not prescribed it.

Methimazole is taken 3 times daily, every 8 hours. The recommended starting total dose of methimazole is 15 mg daily for mild hyperthyroidism, 30 mg to 40 mg daily for moderately severe hyperthyroidism, and 60 mg daily for severe hyperthyroidism. The eventual maintenance dose is normally 5 mg to 15 mg daily.

Methimazole can be taken with or without food.

Many things can affect the dose of medication that a person needs, such as body weight, other medical conditions, and other medications. If your doctor has recommended a dose different from the ones listed here, do not change the way that you are taking the medication without consulting your doctor or pharmacist.

It is important to take this medication exactly as prescribed by your doctor. The 3 daily doses should be taken as close to every 8 hours as possible so that blood levels are kept as constant as possible. If you miss a dose of this medication, take it as soon as possible and check with your doctor or pharmacist.

Store this medication at room temperature, protect it from light and moisture, and keep it out of the reach of children.

5 mg
Each white-to-off-white, round tablet, debossed with “MET and 5” separated by breakline on one side and plain on other side, contains 5 mg of methimazole. Nonmedicinal ingredients: pregelatinised starch, lactose monohydrate, magnesium stearate, and talc.

10 mg
Each white-to-off-white, round tablet, debossed with “MET and 10” on one side and plain on other side, contains 10 mg of methimazole. Nonmedicinal ingredients: pregelatinised starch, lactose monohydrate, magnesium stearate, and talc.

Methimazole should not be taken by anyone who:

• is allergic to methimazole or to any of the ingredients of the medication
• have a history of pancreatitis after taking methimazole in the past
• is breast-feeding

Many medications can cause side effects. A side effect is an unwanted response to a medication when it is taken in normal doses. Side effects can be mild or severe, temporary or permanent. The side effects listed below are not experienced by everyone who takes this medication. If you are concerned about side effects, discuss the risks and benefits of this medication with your doctor.

The following side effects have been reported by at least 1% of people taking this medication. Many of these side effects can be managed, and some may go away on their own over time.

Contact your doctor if you experience these side effects and they are severe or bothersome. Your pharmacist may be able to advise you on managing side effects.

• change in skin colour
• dizziness
• drowsiness
• hair loss
• headache
• heartburn
• loss of appetite
• loss of taste sensation
• nausea
• skin rash or itching
• stomach pain
• vomiting

Although most of these side effects listed below don't happen very often, they could lead to serious problems if you do not check with your doctor or seek medical attention.

Check with your doctor as soon as possible if any of the following side effects occur:

• fever
• itching and hives
• kidney inflammation (e.g., backache, decreased urine, cloudy or bloody urine)
• lupus-like symptoms (e.g., fever, general feeling of being unwell, joint pain, confusion, muscle aches, skin rash)
• muscle or joint pain
• numbness or tingling of fingers, toes, or face
• spinning sensation
• signs of anemia (low red blood cells; e.g., dizziness, pale skin, unusual tiredness or weakness, shortness of breath)
• signs of blood vessel inflammation (e.g., fever, headache, tiredness, weight loss, night sweats, rash, blood in urine, coughing up blood, shortness of breath)
• signs of clotting problems (e.g., unusual nosebleeds, bruising, blood in urine, coughing blood, bleeding gums, cuts that don't stop bleeding)
• swelling of feet or lower legs
• swollen lymph nodes
• symptoms of liver damage (e.g., yellow skin or eyes, abdominal pain, dark urine, clay-coloured stools, loss of appetite, nausea and vomiting, or itching)
• symptoms of low blood sugar (e.g., cold sweat, cool pale skin, headache, fast heartbeat, weakness)

Stop taking the medication and seek immediate medical attention if any of the following occur:

• increased frequency of infections (fever or chills, severe diarrhea, shortness of breath, prolonged dizziness, headache, stiff neck, weight loss, or listlessness)
• severe skin rash
• signs of pancreatitis (e.g., abdominal pain on the upper left side, back pain, nausea, fever, chills, rapid heartbeat, swollen abdomen)

Some people may experience side effects other than those listed. Check with your doctor if you notice any symptom that worries you while you are taking this medication.

Before you begin using a medication, be sure to inform your doctor of any medical conditions or allergies you may have, any medications you are taking, whether you are pregnant or breast-feeding, and any other significant facts about your health. These factors may affect how you should use this medication.

Bleeding: Methimazole may cause a reduced number of platelets in the blood, which can make it difficult to stop cuts from bleeding. If you notice any signs of bleeding, such as frequent nosebleeds, unexplained bruising, or black and tarry stools, notify your doctor as soon as possible. Your doctor will order routine blood tests to make sure potential problems are caught early.

Drowsiness/dizziness: Methimazole may affect the mental or physical abilities needed to drive or operate machinery by causing tiredness or spinning sensations. Avoid driving, operating machinery, or performing other potentially hazardous tasks until you have determined how you are affected by this medication.

Galactose intolerance/glucose malabsorption: Methimazole medications are prepared with lactose. If you have lactose or galactose intolerance you should not take these medications.

Infection: Methimazole can reduce the number of cells that fight infection in the body (white blood cells). Your doctor will likely want you to have laboratory tests done on a regular basis because in rare cases, methimazole can cause the levels of white blood cells to decrease. Low white blood cell levels can increase the risk of infection. If you have a sore throat, skin rash, fever, headache, or a general feeling of being unwell, tell your doctor immediately.

Liver function: This medication can cause liver damage. If you have liver problems, discuss with your doctor how this medication may affect your medical condition, how your medical condition may affect the dosing and effectiveness of this medication, and whether any special monitoring is needed.

Pancreatitis: Although uncommon, methimazole can cause the pancreas to become inflamed. If you have a history of pancreatitis, discuss with your doctor how this medication may affect your medical condition, how your medical condition may affect the dosing and effectiveness of this medication, and whether any special monitoring is needed.

Report signs of pancreatitis such as abdominal pain on the upper left side, back pain, nausea, fever, chills, rapid heartbeat, or swollen abdomen to your doctor immediately.

Thyroid function: Methimazole can cause symptoms of decreased thyroid function if the dose is too high. If you experience symptoms of underactive thyroid, such as unexpectedly feeling cold, depression, constipation, or fatigue, let your doctor know.

Pregnancy: This medication should not be used during pregnancy unless the benefits outweigh the risks. If you become pregnant while taking this medication, contact your doctor immediately.

Breast-feeding: This medication passes into breast milk. If you are breast-feeding and taking methimazole, it may affect your baby. People using methimazole are advised to not breast-feed.

There may be an interaction between methimazole and any of the following:

• BCG intravesical
• cladribine
• clozapine
• deferiprone
• digoxin
• mesalamine
• olaparib
• olsalazine
• prednisolone
• promazine
• sodium iodide
• sulfasalazine
• theophyllines (e.g., aminophylline, oxtriphylline, theophylline)
• warfarin

If you are taking any of these medications, speak with your doctor or pharmacist. Depending on your specific circumstances, your doctor may want you to:

• stop taking one of the medications,
• change one of the medications to another,
• change how you are taking one or both of the medications, or
• leave everything as is.

An interaction between two medications does not always mean that you must stop taking one of them. Speak to your doctor about how any drug interactions are being managed or should be managed.

Medications other than those listed above may interact with this medication. Tell your doctor or prescriber about all prescription, over-the-counter (non-prescription), and herbal medications you are taking. Also tell them about any supplements you take. Since caffeine, alcohol, the nicotine from cigarettes, or street drugs can affect the action of many medications, you should let your prescriber know if you use them.