Adverse drug reactions are a major health problem in the United States. About 25% of all adverse drug reactions have an allergic, pseudoallergic, or idiosyncratic/intolerant basis. Idiosyncratic drug reactions and drug intolerance are also included in this category. Drug allergy may be classified by the Gell and Coombs classification of human hypersensitivity (Type 1: IgE-mediated; Type 2: cytotoxic; Type 3: immune complex; and Type 4: cellular immune mediated). Drug allergy is also frequently characterized by the predominant tissue/organ involved (systemic, cutaneous, or visceral). To some extent, the structural characteristics of drugs and biologic products predict the type of hypersensitivity reaction.