Understanding clinical trial phases

What Phase 1, 2, 3 and 4 trials mean, and what each phase is designed to learn.

4 min read · Informational only, not medical advice.

Phase 1

Small studies, often 20–80 people, that focus on safety and finding the right dose.

Larger studies, often a few hundred people, that look at whether a treatment works and continue to monitor safety.

Studies with hundreds to thousands of participants that compare a new treatment with the current standard of care.

Studies run after a treatment is approved to learn more about long-term benefits and risks.

Some studies — such as behavioral or device studies — don't use phases. Listings show these as 'Not applicable'.

What is a Phase 1 clinical trial?: Small studies, often 20–80 people, that focus on safety and finding the right dose.
What is a Phase 2 clinical trial?: Larger studies, often a few hundred people, that look at whether a treatment works and continue to monitor safety.
What is a Phase 3 clinical trial?: Studies with hundreds to thousands of participants that compare a new treatment with the current standard of care.
What is a Phase 4 clinical trial?: Studies run after a treatment is approved to learn more about long-term benefits and risks.
Why do some trials have no phase?: Some studies — such as behavioral or device studies — don't use phases. Listings show these as 'Not applicable'.

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