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Study phases, explained

Trials that test a treatment usually move through phases, each answering a bigger question.

Last reviewed 14 September 2026

The phases

Early Phase 1

Very small, exploratory studies, sometimes with very low doses, before a full Phase 1.

Phase 1

Usually a small group. The main questions are about safety, the right dose, and how the body handles the treatment.

Phase 2

A larger group, often people with the condition. The focus is whether the treatment has an effect, while safety is watched closely.

Phase 3

Larger still, often in many places. The treatment is compared with the usual care or a placebo to confirm whether it works and to spot less common side effects.

Phase 4

After a treatment is approved. These studies follow how it performs in everyday use, over a longer time.

Studies without a phase

Observational studies, and many studies of devices or behaviour, don't use phases. On our listings they show as “Not applicable”.

This is general information, not medical advice. Talk to your doctor about whether a study could be right for you, and to the study team about the details of a specific study.

Sources

Written from public information published by ClinicalTrials.gov (U.S. National Library of Medicine) and the U.S. National Institutes of Health.