Clinical trial phases

Drug development is conventionally described in numbered phases: first human exposure and dose-finding, then a first look for effect in the target disease, then a larger confirmatory comparison, and then study that continues after approval. The numbers describe decisions rather than a timetable — each phase exists to answer the question that justifies paying for the next one.

We report phases because the label is often used as a shorthand for certainty and is not one. A Phase 2 result is a reason to run a Phase 3 trial, not a reason to expect a medicine; and the phase model itself fits vaccines, biologics and cell therapies only loosely.