Most hypersensitivity reactions to drugs occur within several weeks of administration; signs and symptoms are often consistent with known immune-mediated reactions, including anaphylaxis, rashes, fever, cytopenias and vasculitis. The culprit immune mechanisms range from immunoglobulin E antibody to T cells inducing apoptosis of keratinocytes, in the case of bullous exfoliative rashes. Many drugs induce reactions via altered hepatic metabolism, with production of reactive intermediates which induce a common syndrome of rash and fever plus variable types of other signs. Examples of this reactive metabolite syndrome include the rash and fever in HIV-positive patients given sulfamethoxazole and reactions to the aromatic anticonvulsants. With the notable exception of anaphylaxis and severe bullous exfoliative rashes, most immune reactions to drugs are not life-threatening and generally resolve once the drug is discontinued. The key is prevention. Specific immune testing is standardized only for penicillin. If test results are negative, however, the patient can tolerate all beta-lactam antibiotics. Of those patients with a positive penicillin skin test, only 2% develop reactions when given cephalosporins. Sulfa and quinolone antibiotics, and muscle relaxants, also frequently induce reactions. If there is a history of bullous rash, the patient should never again receive sulfa or quinolone, or related drugs. In other cases, a cautious graded challenge or desensitization can be done. Vancomycin, protamine, and radiocontrast media induce non-immune reactions secondary to their irritant effects on vascular endothelium. Narcotic pain medications cause histamine release by binding to a specific receptor on mast cells in sensitive patients. In contrast to true immune reactions, most patients can receive these medications again, if they are pretreated and the drugs are given slowly. Angiotensin-converting enzymes, aspirin, and non-steroidal anti-inflammatory drugs induce adverse reactions by their effect on enzymes. Readministration usually results in repeat symptoms. It is possible to desensitize patients to aspirin. Some patients appear to develop similar adverse symptoms with multiple unrelated drugs. Although these cases present management problems, most patients can complete a therapeutic course of a vital drug, after careful review of the history, immune testing when possible, and graded challenge or desensitization.