Understanding clinical trial phases

Clinical trials happen in phases. Each phase answers a different question, and a treatment normally has to pass each phase before moving on to the next.

Phase 1 is the first time a new treatment is tested in people. It is usually small — sometimes only a few dozen volunteers — and looks mainly at safety and the right dose.

Phase 2 tests whether the treatment seems to work for the condition it's designed for. It also looks for side effects in more people.

Phase 3 compares the new treatment with the current standard of care, often in hundreds or thousands of people. This is the phase that usually leads to approval.

Phase 4 happens after a treatment is approved. It watches how it performs in everyday use and picks up rare side effects.