Much quantitative information on the clinical use and adverse effects of drugs has been accumulated in the past 15 years. These data provide a basis for substantial knowledge and perspective on the problem of drug toxicity, particularly for drugs that have been marketed for many years. We have also learned how to study adverse effects of newly marketed drugs in a cost-efficient and effective manner. Studies have shown that hospitalized adult patients in the United States receive on average about 10 drugs during hospitalization. Outpatient adults take on average about two drugs on a regular basis (i.e., at least once a week). Results from in-hospital studies indicate that some type of adverse reaction occurs about once in every 20 drug treatments. Most of these drug reactions are clinically minor and reversible, but about 10% of adverse reactions are regarded as life-threatening. Virtually all organ systems have been involved with adverse drug effects. Deaths in hospitalized patients that are attributable to drugs are rare and tend to occur in patients who are quite ill from their primary illness and receive potent drugs such as antitumor agents. However, some drug-induced illnesses appear to be preventable. Out-patient drug toxicity is the cause of about 3% of all hospitalizations. When considered with the enormous use of drugs in the outpatient population, however, this number of admissions secondary to drug toxicity seems to represent a small incidence.