Methimazole adverse effects

Adverse effects occur in 3-12% in the form of rash, fever, urticaria, vasculitis, arthralgia, a lupuslike reaction, cholestatic jaundice, hepatitis, lym-phadenopathy and polyserositis but the most dangerous adverse effect is agranulocytosis (it occurs only in 0.3-0.6%). The reaction is readily reversible when the drug is discontinued. Cross-sensitivity between propylthiouracil and methimazole is about 50%, therefore switching drugs in patients with severe reactions is not recommended. 

Adverse reactions to the thioamides occur in 3-12% of treated patients. Most reactions occur early, especially nausea and gastrointestinal distress. An altered sense of taste or smell may occur with methimazole. The most common adverse effect is a maculopapular pruritic rash (4-6%), at times accompanied by systemic signs such as fever. Rare adverse effects include an urticarial rash, vasculitis, a lupus-like reaction, lymphadenopathy, hypoprothrombinemia, exfoliative dermatitis, polyserositis, and acute arthralgia. Hepatitis (more common with propylthiouracil) and cholestatic jaundice (more common with methimazole) can be fatal, although asymptomatic elevations in transaminase levels also occur. 

Di Gianantonio E, Schaefer C, Mastroiacovo PP, Cournot MP, Benedicenti F, Reuvers M, Occupati B, Robert E, Bellemin B, Addis A, Arnon J, dementi M. Adverse effects of prenatal methimazole exposure. Teratology 2001 64(5) 262-6. 

Adverse reactions The most common adverse effect is maculopapular rash. Rarely, hepatitis, vasculitis, urticarial rash, and arthralgia have been observed. Agranulocytosis can occur in 0.3% to 0.6% of patients. Patients who develop agranulocytosis with one thioamide should not be switched to the alternate thioamide because there is a 50% cross-reactivity between the agents. Methimazole is contraindicated during pregnancy because scalp defeas have been observed in infants bom to mothers using methimazole. 

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