Methimazole Side Effects

Please note - some side effects for Methimazole may not be reported. Always consult your doctor or healthcare specialist for medical advice. You may also report side effects to the FDA.

Side Effects of Methimazole - for the Consumer

Methimazole

All medicines may cause side effects, but many people have no, or minor, side effects. Check with your doctor if any of these most COMMON side effects persist or become bothersome when using Methimazole:

Change in taste; dizziness; drowsiness; headache; joint pain; lightheadedness; muscle pain; nausea; numbness and tingling; stomach upset; vomiting.

Severe allergic reactions (rash; hives; itching; difficulty breathing; tightness in the chest; swelling of the mouth, face, lips, or tongue); changes in appetite; dark urine; fainting; fast or irregular heartbeat; fever, chills, or persistent sore throat; severe or persistent headache, nausea, or vomiting; stomach pain; tremor; unusual bruising or bleeding; unusual hair loss; unusual tiredness or weakness; vision changes; yellowing of the skin or eyes.

This is not a complete list of all side effects that may occur. If you have questions about side effects, contact your health care provider. Call your doctor for medical advice about side effects. To report side effects to the appropriate agency, please read the Guide to Reporting Problems to FDA.

Methimazole Side Effects - for the Professional

Methimazole

Major adverse reactions (which occur with much less frequency than the minor adverse reactions) include inhibition of myelopoiesis (agranulocytosis, granulocytopenia, and thrombocytopenia), aplastic anemia, drug fever, a lupuslike syndrome, insulin autoimmune syndrome (which can result in hypoglycemic coma), hepatitis (jaundice may persist for several weeks after discontinuation of the drug), periarteritis, and hypoprothrombinemia. Nephritis occurs very rarely.

Minor adverse reactions include skin rash, urticaria, nausea, vomiting, epigastric distress, arthralgia, paresthesia, loss of taste, abnormal loss of hair, myalgia, headache, pruritus, drowsiness, neuritis, edema, vertigo, skin pigmentation, jaundice, sialadenopathy, and lymphadenopathy.

It should be noted that about 10% of patients with untreated hyperthyroidism have leukopenia (white-blood-cell count of less than 4,000/mm3), often with relative granulopenia.

Side Effects by Body System - for Healthcare Professionals

Hematologic

The onset of aplastic anemia (pancytopenia) or agranulocytosis may occur in hours to days. Most cases of agranulocytosis occur within the first 90 days of treatment, but this complication can occur even a year or more after starting therapy.

Studies have suggested that the risk of agranulocytosis is greater in older patients and that they have a higher rate of death.

Hematologic side effects have included inhibition of myelopoiesis (agranulocytosis (0.35%), granulocytopenia, and thrombocytopenia), aplastic anemia, and hypoprothrombinemia. Approximately 10% of patients with untreated hyperthyroidism have experienced leukopenia, often with relative granulopenia.

Immunologic

Immunologic side effects have included insulin autoimmune syndrome and lupus-like syndrome.

Hepatic

Hepatic side effects have included hepatitis and jaundice (which may persist for several weeks following discontinuation of the drug).

Renal

Renal side effects have rarely included nephritis.

Dermatologic

A 5-year-old female experienced aplasia cutis congenita coincident with in utero exposure to methimazole. She presented with hairless scars on her scalp. At birth the lesions were friable and bled easily, however, over time they became scar-like and thicker. At the time of conception, her mother was receiving therapy for Graves' disease with methimazole 20 mg daily. Aplasia cutis congenita has been reported as a possible teratogenic effect of methimazole therapy in multiple case reports. However, the causal relationship between the therapy and the skin defect is controversial, and anomalies related to methimazole have low birth prevalence.

Dermatologic side effects including urticaria (5%), macular rash (5%), rash, pruritus, abnormal loss of hair, and skin pigmentation have been reported. Aplasia cutis congenita has also been reported.

Musculoskeletal

Musculoskeletal side effects have included arthralgia (5%) and myalgia.

Nervous system

Nervous system side effects have included paresthesia, headache, drowsiness, neuritis, and vertigo.

Gastrointestinal

Gastrointestinal (GI) side effects have included GI upset (5%), nausea, vomiting, epigastric distress, and taste disturbances. Pancreatitis has been reported very rarely.

Ocular

Ocular side effects including at least one case of carotid cavernous fistula have been reported.

A 67-year-old female with a history of Graves' disease and mild bilateral ophthalmopathy experienced carotid cavernous fistula coincident with methimazole therapy. She was treated with radioiodine following 10-years of therapy with methimazole. Evaluation of the patient with sensitive techniques lead to the diagnosis of carotid cavernous fistula. This is the first report of the simultaneous occurrence of Graves' ophthalmopathy and carotid cavernous fistula.

Cardiovascular

Cardiovascular side effects including vasculitis have been reported.

Metabolic

Metabolic side effects including rare reports of hypoglycemia have been reported.

Other

Other side effects have included edema, periarteritis, sialadenopathy, lymphadenopathy, and fever. Abnormal sense of taste or smell has also been reported rarely.

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