Methimazole

Medically Reviewed by Dr. Harshi Dhingra, M.D.

Last Reviewed: Aug 16, 2023

Warnings

Methimazole Risks, Warnings, and Complications

Methimazole can cause or worsen health problems in some people. Talk to your doctor or pharmacist about other treatments for hyperthyroidism if you are at a higher risk of adverse reactions or serious side effects from this medication. Some of the risks of taking methimazole include:

AGRANULOCYTOSIS: Taking methimazole can lead to lower white blood cells. This is called agranulocytosis. It can also cause aplastic anemia (pancytopenia), and thrombocytopenia (low platelets). Agranulocytosis can be life-threatening because white blood cells are needed to help the body fight infections. Before starting methimazole, tell your healthcare provider if you are on other drugs that can also affect immune function, such as steroid medicines, or if you have health conditions that affect the immune system. Call your doctor immediately if you develop signs and symptoms of infection, such as fever, chills, or sore throat. Also, call your doctor if you develop fatigue, weakness, frequent infections, or unusual bruising or bleeding while on methimazole.
LIVER PROBLEMS: Methimazole can cause liver damage. The risk is higher in people with a history of alcohol use or pre-existing liver disease. Call your doctor immediately if you develop signs and symptoms that may be related to liver dysfunction, such as loss of appetite, itching, right upper stomach pain, nausea, vomiting, yellowing of the skin or whites of the eyes, or dark urine. You may need to stop taking methimazole.
HYPOTHYROIDISM: Methimazole works by interfering with the formation of thyroid hormones. It can, therefore, sometimes lead to too little thyroid hormone (hypothyroidism). Call your doctor if you develop symptoms such as fatigue, weakness, dry skin, or constipation. Your doctor will check thyroid hormone levels and adjust your dose of methimazole if needed.
RISK DURING PREGNANCY AND LACTATION: Methimazole can cause harm to the unborn baby if it is taken by a pregnant woman. Tell your doctor if you are pregnant or become pregnant while on this medicine. Also, tell your doctor if you are nursing an infant with breast milk. Nursing mothers should not take methimazole because this medication passes into breast milk.

Precautions Before Starting Methimazole

Tell your doctor if you have ever had an allergic reaction to methimazole, any of the active or inactive ingredients in methimazole tablets, other antithyroid drugs, or any other medications. Your pharmacy can give you a list of ingredients.

Give your doctor and pharmacist a complete list of your medications, including prescription medications, over-the-counter medicines, vitamins, dietary supplements, and herbal remedies. This can help avoid possible drug interactions between methimazole and your other medicines.

Methimazole may not be right for people with certain medical conditions. Give your doctor a complete medical history. Be sure to mention if you have liver disease (hepatitis, jaundice), aplastic anemia, thrombocytopenia (low platelets), or leukopenia (decreased white blood cell count).

Tell your health care professional if you are pregnant, could be pregnant, plan to get pregnant, or breast feeding.

Precautions During Use of Methimazole

Keep all your medical and lab appointments during methimazole therapy. Follow your doctor’s dosing instructions carefully. Doctors usually order blood tests and adjust the dose of methimazole based on patient response.

Tell your doctor if you become pregnant while on this medication.

Tell all your healthcare providers you are on methimazole, especially before any procedure, including dental procedures.

Methimazole Drug Interactions

Other drugs can affect how methimazole works. Possible interactions with certain medications can increase the risk of severe adverse effects. Some of the medications that can have interactions with methimazole include:

Anticoagulants or blood thinners such as warfarin (Coumadin)
Diabetes medications
Heart medications such as digoxin (Lanoxin)
Asthma medications such as theophylline (Theobid, Theo-Dur)
Beta blockers such as propranolol (Inderal)
Vitamins

Give your doctor or pharmacist a complete list of your medications, including prescription drugs, nonprescription drugs, dietary supplements, and herbal remedies. Also, tell your healthcare professional if you smoke, drink alcohol, or use recreational drugs because some of these substances can cause serious health complications when used with prescription medications.

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