Methimazole is a medication used to treat hyperthyroidism, a condition where the thyroid gland produces too much thyroid hormone. It works by making it harder for the body to make thyroid hormone. Methimazole is also used before thyroid surgery or radioactive iodine treatment. It is available only with a doctors prescription and comes in tablet form. The medication is usually taken three times a day, approximately every 8 hours, with food. The dosage is based on weight in children. Methimazole can cause side effects such as decreased white blood cells, decreased platelets, and liver disease. If you experience any unusual symptoms, you should contact your doctor immediately.