It is impossible to generalize how long it takes to develop an allergy to taking methimazole. It is possible for some patients to develop an allergy within 1-2 weeks, or even later, and most patients do not develop an allergy. Methimazole belongs to the antithyroid drugs, suitable for all types of hyperthyroidism, allergic reaction is a more common adverse reaction of the drug, due to individual differences, the specific time of allergy, can not be determined. In general, most patients do not experience allergic reactions, and some patients may experience allergies within 1-2 weeks or later. The allergy is mostly a mild reaction that will gradually disappear with dosing. In addition to this, methimazole has the potential to cause adverse reactions such as granulocyte deficiency, arthralgia, and myalgia. Previous use of carbimazole or methimazole acute pancreatitis, moderate to severe blood count disorders (neutropenia), are prohibited. Methimazole is a prescription drug and must be used in strict accordance with medical advice.