Trial phases
How the four phases of clinical research differ
Each phase asks a different question. Use the explorer or the comparison table to see how goals and visits change.
Phase explorer
Phase I Studies
Learn how a new approach behaves in the body and what amount is well tolerated.
- Who takes part
- Often a small group, sometimes healthy volunteers
- Study visits
- Visits can be frequent and may include overnight stays at a research unit.
- Tolerability and safety
- How the body processes a study product
- Dose ranges
Comparison guide
| Phase | Main goal | Who takes part | Visits |
|---|---|---|---|
| Phase I Studies | Learn how a new approach behaves in the body and what amount is well tolerated. | Often a small group, sometimes healthy volunteers | Visits can be frequent and may include overnight stays at a research unit. |
| Phase II Studies | Look at whether an approach shows signs of working for a specific condition while continuing to study safety. | A larger group of people who have the condition being studied | Visits are usually scheduled over weeks or months with regular check ins. |
| Phase III Studies | Compare a new approach with current standard options across a broad group of people. | Hundreds to thousands of volunteers, often at many locations | Visits may follow a set schedule over a longer period, sometimes with remote options. |
| Phase IV Studies | Follow an approach after it has been approved to learn about long term use in everyday settings. | Large and varied groups of people | Participation can be lighter, sometimes based on surveys or routine visits. |