"Regulatory agencies approve drugs for marketing when the ratio of risk to benefit of the drugs is positive, i.e. the potential benefit of using the drug justifies the risk of drug related adverse reactions (ADR). At the point of approval of a drug there is not adequate information about all possible negative outcomes arising from its use in clinical practice. The information is even more limited for patient subgroups, diseases and treatment combinations that were not evaluated in the clinical trials required for approval. Therefore, performing post-marketing pharmacoepidemiological studies is necessary for a better understanding of drug use in usual clinical practice conditions and the effect of the drugs on the general patient population."
Seoane-Vazquez E. Drug selection and adverse drug reactions: Balancing outcomes and costs. Electronic Journal of Biomedicine. 2008;2:3-5.