CancerExplained.org · Free health handout
Understanding Trial Phases
What Phase 1, 2, 3, and 4 actually mean
Phase 1 — Is it safe?
- Small groups of people (often 15–50).
- Finds a safe dose, how the treatment should be given, and what side effects appear.
- Participants are monitored very closely.
Phase 2 — Does it work?
- Usually fewer than 100 people with a specific cancer type.
- Studies whether the treatment has an effect against that cancer while continuing to track safety.
Phase 3 — Is it better than what we have?
- Hundreds to thousands of participants, often at many hospitals.
- Compares the new approach with the current standard treatment, usually by random assignment.
- Positive Phase 3 results are the usual basis for FDA approval.
Phase 4 — What happens long-term?
- Runs after a treatment is approved and in use.
- Tracks long-term safety, side effects, and benefits in much larger groups.
Each phase answers a different question — ask your care team which phase a trial is in and what that means for you.
This handout is for education only and is not medical advice. Talk with a qualified healthcare professional about what is right for you.
Sources: NCI: What are clinical trials? · NCI: Clinical trials information. Updated 2026-07-05.
Learn more in plain language: https://cancerexplained.org/clinical-trials/how-they-work/ — free to copy and share for non-commercial education.