Clinical trial phases
Phases of clinical trials explained
New treatments move through phases, each answering a different question. Each phase only starts once the previous one shows it is safe to continue.
Phase 0
A handful of people
Tiny doses to see how a drug behaves in the body. No treatment benefit expected.
Phase 1
20–100, often healthy volunteers
Safety, dosing and how the body absorbs and clears the medicine. Bioequivalence (BA/BE) studies are similar.
Phase 2
Up to a few hundred patients
Whether the treatment works for the condition and which side effects appear.
Phase 3
Hundreds to thousands of patients
Confirms benefit and safety against standard treatment; the basis for approval.
Phase 4
Large groups after approval
Long-term safety and real-world use once the medicine is on the market.
Want the full guide? Read our phases article · Healthy volunteer studies · Paid clinical trials