01 Why phases exist
Testing a new medicine step by step lets researchers learn about safety before exposing larger groups of people. Each phase builds on the one before. If an earlier phase raises concerns, later ones may not happen.
02 Phase 0 (sometimes)
A very small, very short study that gives a tiny dose to learn how the body handles a substance. It is not designed to treat anyone. Many medicines skip it.
03 Phase I: is it safe?
A small group, often from a few dozen people, takes the treatment so researchers can look for side effects and work out a safe dose range. Some Phase I studies enroll healthy volunteers, and others enroll people with a specific condition.
04 Phase II: does it seem to work?
A larger group, often up to a few hundred, takes the treatment so researchers can see whether it has an effect on the condition and keep watching for side effects. Some Phase II studies compare the treatment to a placebo or to standard care.
05 Phase III: how does it compare?
The largest studies, often hundreds to thousands of people at many sites, compare the new approach with the current standard. Results from Phase III studies are what regulators usually weigh most heavily when deciding whether to approve a treatment.
06 Phase IV: what happens afterward?
After approval, researchers can keep studying a treatment in wider groups and over longer periods to learn about rare side effects and long-term results.
07 What the numbers don’t tell you
A higher phase does not mean a study is “better,” and an early phase does not mean it is risky in every way. The right question is whether the purpose, the possible risks, and the time commitment make sense for you, which is something to talk through with your own doctor and the study team.
Studies of devices, diagnostics, behavior, and care delivery often do not use phases. You may see “N/A” for the phase on a listing. That is normal.
General information only. This guide is not medical, legal, or financial advice, and Clintivo is not a medical provider. Talk with your own doctor about your health.