Allergic reaction of the skin and membrane caused by injection, oral administration and inhalation of the drugs into the body is called drug rashes. The commonly encountered allergic drugs are analgesic-antipyretic, sulfanilamide, hypnotic, sedative, and antibiotic, etc. In the recent years, because of the extensive application of Chinese herbal medicines and reform of the herbal doses, the related allergic reaction increases gradually, too.The pathogenesis of drug rashes is complicated. In addition to type I -IV allergic reaction, there are also light sensitive type and fixed drug rash, and undefined type of lupus erythematosus syndrome. Non-allergic pathogenesis also exists in drug rash.