Adverse drug reactions are a major clinical problem affecting up to 20% of hospitalized patients and 10% of the general population [1]. Drug allergic reactions account for about 10% of all adverse drug reactions, which are broadly categorized as type A or type B. Type A adverse drug reactions are mediated by recognized pharmacologic drug effects, that are generally dose-dependent and include side effects and overdose. Type B reactions account for less than 20% of adverse drug reactions and include drug hypersensitivities. While type B reactions were originally described as unpredictable and idiosyncratic with an immunologic basis, many of these features have been called into question.