Phase I, II, III – what the numbers mean
The higher the phase, the more is known about a drug. What is tested in each phase.
When you read a trial description you almost always come across a phase: phase I, II or III. The number tells you how far the development of a drug has progressed – and what is being tested in the trial.
Phase I: safety and dosage
In phase I trials a drug is used in humans for the first time, usually with few participants. The focus is on tolerability and the right dose. In oncology these trials often involve patients for whom no standard treatment is available any more.
Phase II: efficacy
Phase II trials test in a larger group whether the drug works in a particular disease. A comparison group is often already used.
Phase III: comparison with the standard
In phase III the new treatment is compared with the current standard therapy in many participants. The result usually determines whether a drug is approved.
Phase IV: after approval
Observation continues after approval – for example rare side effects or the effect in particular groups. These are phase IV trials.
This information is not a substitute for a conversation with your doctor.