Clinical trial phases

The staged sequence of human studies a new drug must pass, from small safety tests to large efficacy trials, before approval.

• Phase 1 tests safety and dosing in typically 20–100 people; Phase 2 looks for signs of benefit in up to a few hundred patients.
• Phase 3 enrols hundreds to thousands, randomly assigned to the drug or to placebo or standard care, to prove benefit with statistical confidence.
• Roughly 1 in 10 drugs entering Phase 1 is approved; Phase 2 is the steepest drop, where most candidates fail to show they work.
• A full programme often takes 6–10 years and hundreds of millions of dollars, so a small positive early trial is weak evidence.