methimazole, Tapazole, Northyx (cont.)

Pharmacy Author:
Medical and Pharmacy Editor:
Medical Editor:

DOSING: The initial adult dose of methimazole is:

  • 15 mg/day for mild hyperthyroidism
  • 30-40 mg/day for moderately severe hyperthyroidism
  • 60 mg/day for severe hyperthyroidism

The daily dose is divided into three doses administered every 8 hours.

The maintenance dose is 5-15 mg/day.

The usual initial children's dose is 0.4 mg/kg given in 3 divided doses administered 8 hours apart, and the maintenance dose is half the initial dose.

DRUG INTERACTIONS: Warfarin (Coumadin) works by reducing the activity of vitamin K and therefore the formation of vitamin K dependent clotting factors. Methimazole may increase the activity of warfarin by further reducing the activity of vitamin K in the body.

Hyperthyroidism increases elimination of beta blockers. Correcting hyperthyroidism will cause less elimination of beta blockers. Therefore, the dose of beta blockers may require reduction when hyperthyroidism is corrected. A similar reaction may also occur with digoxin (Lanoxin) and theophylline (Respbid, Slo-Bid, Theo-24, Theolair).

PREGNANCY: There is evidence that methimazole may cause harm to the fetus during pregnancy.

NURSING MOTHERS: Methimazole is excreted in breast milk and may potentially cause harm to the infant.

SIDE EFFECTS: Methimazole is generally well-tolerated with side effects occurring in 3 out of every 100 patients. The most common side effects are related to the skin and include rash, itching, hives, abnormal hair loss, and skin pigmentation. Other common side effects are swelling, nausea, vomiting, heartburn, loss of taste, joint or muscle aches, numbness and headache.

Medically Reviewed by a Doctor on 8/6/2014


Report Problems to the Food and Drug Administration

You are encouraged to report negative side effects of prescription drugs to the FDA. Visit the FDA MedWatch website or call 1-800-FDA-1088.

Pill Finder Tool

Need help identifying pills and medications?
Use the pill identifier tool on RxList.


Back to Medications Index