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The role of metabolites in bioequivalence studies has been a contentious issue for many years. Many papers have published recommendations for the use of metabolite data based on anecdotal evidence from the results of bioequivalence studies. Such anecdotal evidence has validity, but the arguments lack weight because the “correct” answers are always unknown. A more promising area of exploration is recommendations based on simulated bioequivalence studies for which the “correct” answers are known, given the assumptions. A review of the literature, however, reveals scant evidence of attempts to apply to real data the pharmacokinetic principles on which the recommendations from simulated studies relied. We therefore applied those principles (based on estimates of intrinsic clearance after oral administration of the parent drug) to four bioequivalence studies from our archives, in which the parent drug and at least one metabolite were monitored. In each case, the outcome is discussed in the context of the complexity of the metabolic processes that impact on the parent drug and the metabolite(s) during the first passage from the intestinal lumen to the systemic circulation. Our observation is that no simple generalization can be made such that each drug/metabolite combination must be examined individually. Our recommendation, however, is that in the interests of safety, bioequivalence decision-making should be based on the parent drug whenever possible. [ABSTRACT FROM AUTHOR]