The so-called drug cross-allergy means that the patient has already had an allergic reaction to a drug, and another drug used later, although different from the drug to which the allergy first occurred, but similar in chemical structure to the drug to which the allergy first occurred, will also cause a drug allergic reaction. If a patient has already had an allergic reaction to a certain sulfa drug, then other iodamines should not be used. It is also important to note that some drugs, although not in the sulfonamide class, may cause cross-allergy in people who are allergic to sulfonamides, such as sulfonylurea oral hypoglycemic drugs like metformin tablets, toluenesulfonylurea tablets, and chlorosulfonylurea tablets. Sulfonamides contain a chemical group, p-aminobenzenesulfonamide, and the above drugs also contain a p-aminobenzenesulfonamide group, so drug cross-allergy can occur.