Assessment of patients in whom an allergic drug reaction is suspected should include a carefully taken drug history. This should be followed by diagnostic testing if it is available for the drug in question. If neither an alternative therapeutic agent nor a diagnostic test is available, then a provocation challenge or desensitization may be necessary. Increased understanding of allergic drug reactions requires the following: better pharmacoepidemiologic studies, improved in vitro and in vivo methods of studying the immune responses involved, and expanded comparisons between the pharmacokinetics and pharmacodynamic effects of drugs and their metabolites. Until improvements are made in these areas, the appropriate approach is to avoid the unnecessary use of drugs. When a drug is required, patient risk factors for an allergic reaction must always be weighed against the benefits of treatment.