OVER 13,000 drugs are listed in thePhysicians' Desk Reference.1Although some vary in compounding, preparation, or brand name, the number of different drugs is staggering. No drug is completely harmless, even when used correctly, and all may produce adverse reactions. With rare exceptions, however, the exact incidence of adverse reactions to any of the drugs is not known. Based on sporadic reports of such reactions, and drawing from experience and observations, the physician empirically evaluates risks and hopes to avoid serious adverse drug reactions in his patients. Nevertheless, drugs do occasionally produce severe illness and death. Most often, adverse drug reactions are not severe, but the predictability of seriousness usually is not possible, and when reactions occur they are unexpected. The circumstances and factors predisposing patients to reactions, for the most part, are undefined. Previous observations suggest that certain individuals or certain diseases may be associated with increased