Here, we reported two cases with hyperthyroidism who complained of myalgia and muscle cramps during treatment with methimazole tablets (or Thyrozol, the brand name). One case experienced muscle cramps after taking Thyrozol for 6 months, and by this time the patient's thyroid function had returned to normal. In the other case, pain caused by muscular cramps began after the patient took Thyrozol for two weeks and the patient's thyroid function had not returned to normal yet at the time. In both cases, pain caused by muscle cramps appeared while the patients were taking Thyrozol. The myalgia persisted in spite of a reduction in the Thyrozol dose, but was significantly relieved with the discontinuation of Thyrozol. Myalgia and muscle cramps did not recur after the patients were switched to methimazole ointment. There was a strong temporal association between oral administration of Thyrozol and pain caused by muscle cramps, which may indicate that myalgia and muscle cramps are adverse reactions of Thyrozol. Looking into the relevant literature on the topic, we explored in this report the possible mechanisms of the onset of muscle cramps associated with Thyrozol, and compared the adverse reactions of two different formulations of methimazole, intending to provide more clinical experience for the treatment of hyperthyroidism and the management of rare adverse reactions related to antithyroid drugs.