Despite all the good that prescription drugs do, evidence continues to mount that adverse drug events are a common, costly, and often preventable cause of illness, disability, and even death. The challenge is to appreciate this downside of drug therapy, to define it, and to understand how the problems associated with it can be prevented. More and more data are becoming available concerning the frequency, clinical consequences, and cost of adverse drug events. At a time in which measures of quality and expenditures in the healthcare system are being scrutinized with great care, these are particularly important issues. Perhaps most importantly, adverse drug events are preventable in many instances. For healthcare resources to be used as efficiently as possible, preventing drug induced illness is one of the most promising areas for future efforts. This does not require rationing or withholding of care; it just requires better clinical decision making. In order to accomplish this, it is necessary to understand the causes of drug induced illness.

Most drug-induced illness comes about through one of four mechanisms: (a) poor prescribing decisions by physicians, despite the availability of clear evidence; (b) errors in dispensing or administration of a drug; (c) poor compliance by the patient resulting in under use, overuse, misuse, or complete cessation of therapy; and (d) the occurrence of previously unanticipated adverse drug reactions, whose existence was not clearly predicted by pre-marketing clinical trials. For each of these causes one must consider the origin, its consequences, and, perhaps most important, what can be done for each cause to prevent it.