Anti-thyroid drugs (ATDs), such as methimazole (MMI) and propylthiouracil (PTU), are the most common treatment options for hyperthyroidism. Although effective, well-known adverse effects include agranulocytosis, toxic hepatitis, vasculitis, and arthralgias. Myalgia and elevation of serum creatine kinase (CK) are relatively rare, with an unclear mechanism. Rapid decrease in the thyroid hormone level may be associated with ATD-related myopathy; however, direct effects of the drug on muscle tissue cannot be excluded. Here we report on two Chinese patients with myalgia and an elevated CK due to ATDs. Early recognition of this rare medication-induced adverse effect and close monitoring of the CK level are particularly important. Physicians and pharmacists should inform the patients about the earliest symptoms of adverse effects for patients to know when to discontinue the drug. If adverse events occur, different treatment strategies such as ATD dose reduction and switching to alternative ATDs can be applied depending on the case.