How Clinical Trial Phases Actually Work

Clinical trials are usually described in terms of phases, but a lot of people working adjacent to this industry, in marketing or operations roles for example, never get a clear explanation of what actually separates each phase. This post walks through the purpose of each stage, starting with small phase 1 trials focused mainly on safety, through larger phase 2 and phase 3 trials that test effectiveness and monitor for less common side effects across bigger patient groups. We also touch on what happens after approval, since post marketing surveillance, sometimes called phase 4, continues to track a drug once it is already available to the public. Understanding this progression makes it easier to follow why some trials take years while others move faster.