What are the phases of clinical trials?

Clinical trials of new treatments usually move through a series of phases. Each phase is a separate study that answers different questions about safety, dosing and effectiveness, and a treatment must generally succeed in one phase before it moves to the next.

Phase 1 trials are usually the first time a new treatment is given to people. They involve small groups, often between 20 and 100 participants, and focus on safety: how the treatment behaves in the body, what side effects appear and what dose range looks appropriate. Many phase 1 studies recruit healthy volunteers.

Phase 2 trials involve larger groups, typically up to a few hundred people who have the condition being studied. They continue to monitor safety while looking for early evidence that the treatment works, and they help researchers settle on the best dose.

Phase 3 trials are large studies, often involving hundreds to thousands of participants across many research sites and countries. They compare the new treatment with the current standard of care or a placebo to confirm how well it works and to detect less common side effects. Results from phase 3 trials are usually central to a regulator's decision about approval.

Phase 4 studies happen after a treatment has been approved and is in general use. They track long-term safety and effectiveness in much larger, more diverse populations over time.

Knowing a study's phase helps you understand what is already known about the treatment and what the study is trying to find out. The research team can tell you the phase of any trial you are considering and what that means for you.