Methimazole is contraindicated in patients who are hypersensitive to the drug or any ingredient in the formulation. Cross-sensitivity between thioamides may occur (i.e. in approximately 50% of patients switched from one thioamide agent to the other). In patients who develop agranulocytosis or other serious adverse effects while receiving methimazole, some clinicians state that use of the other drug also is contraindicated because of the risk to cross-sensitivity between the two drugs. In patients experiencing serious allergic reactions to methimazole, some clinicians state that using the alternative antithyroid drug (i.e. propylthiouracil) is not recommended.