Guide
Clinical trial phases explained
New medicines are tested in people in four phases, each larger than the last. A study listing always says which phase it's in, and that tells you a lot about what to expect.
| Phase | Main question | Typical size | Typical length | Share that move on |
|---|---|---|---|---|
| Phase 1 | Is it safe, and what dose works? | 20–100 people, often healthy volunteers | Several months | About 70% |
| Phase 2 | Does it work, and what are the side effects? | Up to several hundred people with the condition | Several months to 2 years | About 33% |
| Phase 3 | Does it work better than current treatment, and is it safe in more people? | 300–3,000 people with the condition | 1–4 years | About 25–30% |
| Phase 4 | How does it perform once approved and widely used? | Several thousand people | Ongoing | — |
Figures from the U.S. Food and Drug Administration.
What this means for you
- Phase 1 studies often involve close monitoring, sometimes with overnight stays. Many recruit healthy volunteers.
- Phase 2 and 3 studies recruit people with the condition being studied. Some participants may get a placebo or the current standard treatment instead of the new one.
- Phase 4 studies happen after a medicine is approved by the FDA, to keep checking its safety over time.
Questions to ask
- Which phase is this study, and what's known about the treatment so far?
- Could I get a placebo?
- What happens when the study ends? Can I keep taking the treatment?
Read more: Clinical trial glossary
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