Evidence-appraisal glossary
Phases of clinical trials
Clinical trial phases are the ordered stages of testing a treatment in humans: phase I checks safety and dose in small groups, phase II explores preliminary efficacy, phase III compares against standard care in larger populations, and phase IV monitors safety after approval. Each phase answers a different question and gates the next.
Also called: trial phases, phase 1 trial, phase 2 trial, phase 3 trial, phase 4 trial.
Early phases prioritize safety and dosing while later phases test whether the treatment actually helps patients, so clearing phase I says little about whether a drug works. Phase III is usually the pivotal evidence regulators weigh for approval, though combined or seamless designs blur the boundaries. The phase label tells you what question a study was built to answer, which shapes how much weight its results can carry.
This is a plain-language methodology definition for reading research. It is general education, not medical advice.