We examined some of the factors that contribute to the development of adverse drug reactions (ADRs) and analyzed postmarketing ADR reports for 22 drugs. The role of metabolic-based drug-drug interaction in the development of ADRs can not be overstated. Assessment of the postmarketing ADR data for 22 drugs revealed that drugs with high potential for eliciting clinically significant ADRs are usually detected and either withdrawn from the market or placed on restricted use within the first year or two of marketing. Postmarketing data could be a useful tool for understanding the ADR profile of drugs if reporting can be adequately monitored and verified. It is hoped that early evaluation of the clinically meaningful factors such as metabolism, pharmacogenetics, and effect of physiologic and pathophysiologic states on the clinical effect of a drug during drug development would significantly reduce the incidence and severity of post-marketing ADRs.