After more than 100 deaths caused by the Elixir Sulfanilamide disaster, in 1938, the U.S. Congress passed the Federal Food, Drug, and Cosmetic Act (FD&C Act) which for the first time in U.S. history required pharmaceutical companies to submit full reports of investigations regarding the safety of new drugs. Not until 1962, after the passage of the Leaver-Harris Amendment of the FD&C Act, was the U.S. Food and Drug Administration (FDA) authorized to require evidence of efficacy for approval of new drugs. Consequently, currently, for approval of a new drug the FDA requires that adequate and well-controlled clinical trials be conducted in humans to demonstrate substantial evidence of effectiveness and safety. In the evolution of regulations on approval of new drugs, therefore, safety of the drug was the primary issue and came before efficacy.However, this may not be totally true for pharmaceutical companies when developing new drugs and for regulatory agencies when approving drugs, at least in design, conduct, and analysis of adequate and well-controlled clinical trials. The current paradigm for approval of a new drug is two-fold. First is to prove that the drug is efficacious. Second is to verify whether there is any excessive safety risk. Most adequate and well-controlled trials are sufficiently powered to evaluate efficacy either by superiority or by noninferiority. However, most so-called "adequate and well-controlled trials" are not powered to detect excessive risk by the following hypothesis:where P T and P C are the proportions of patients with certain adverse events, for test and control groups, respectively. In (1), we use P T ≤ P C simply to illustrate the concept; other parameters for evaluation of risk, such relative risk, odd ratio, or hazard ratios, can also be used.