Important but infrequent adverse drug effects may escape detection in the randomized, controlled trials that demonstrate efficacy and form the basis for approval of drugs for marketing (e.g., COX-2 inhibitors, see Chapter 26). The number of patient-years of drug exposure during a clinical trial is small relative to exposure after the drug is marketed. Furthermore, some adverse effects may have a long latency or may affect patients not included in the controlled trials. Observational studies therefore are used to examine those adverse effects that only become apparent with widespread, prolonged use.
The quality of information derived from observational studies varies with the design and depends highly on the selection of controls and the accuracy of the information on medication use. Cohort studies compare the occurrence of events in users and nonusers of a drug; this is the more powerful of the observational study designs. Case-control studies compare drug exposure among patients with an adverse outcome versus that in control patients. Because the control and treatment groups in an observational study are not selected randomly, they may be different in unknown ways that determine outcome independent of drug use. Because of the limitations of observational studies, their validity cannot be equated with that of randomized, controlled trials (Table 5-1). Observational studies raise questions and pose hypotheses about adverse reactions; if it is not feasible to test these hypotheses in controlled trials, then clinical decisions may be made from replicated findings from observational studies.
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