Phase 1, 2 and 3 trials

For experts

FDA describes Phase 1 as studies in roughly 20 to 100 healthy volunteers or patients focused on safety and how the drug behaves in the body; Phase 2 as studies in up to several hundred patients assessing efficacy signals and side effects; and Phase 3 (pivotal) as studies in about 300 to 3,000 participants that demonstrate whether a treatment benefits a specific population and provide most of the safety data. Phase 4 studies follow approval. The same FDA page says a developer must have adequate data from two large, controlled clinical trials before filing a marketing application; FDA draft guidance also describes how one adequate and well-controlled investigation plus confirmatory evidence can meet the substantial-evidence standard.