Clinical-trial phases explained
Clinical studies are designed around specific questions. A phase number gives useful context, but it is not a verdict by itself and does not replace careful reading of the protocol and results.
By Manchester Peptides · ·
Before human studies
Discovery and preclinical work investigate a candidate before clinical research. Laboratory or animal findings can inform whether human studies should be considered, but they are not substitutes for evidence from people.
Phase 1: initial human information
Early clinical studies commonly focus on safety, tolerability and how a substance behaves in the body. They are usually relatively small. A Phase 1 result cannot answer every question about effectiveness or less common adverse effects.
Phase 2: exploratory efficacy and safety
Phase 2 studies investigate a defined condition or population and collect further safety information. Their design, endpoints, comparator and sample size determine what conclusions are justified.
Phase 3: larger confirmatory studies
Phase 3 studies are generally larger and designed to test benefit and monitor adverse reactions in a specified population. Larger numbers can reveal information that smaller studies may miss, but results still apply to the studied formulation, protocol and participants.
Phase 4: after authorisation
Post-authorisation studies and safety monitoring can examine real-world use, longer-term outcomes or rarer events. Authorisation in one country or for one indication should not be assumed to apply to another product or purpose.
Source and further reading
FDA: Step 3—Clinical Research. The source explains the US process; UK decisions are made under UK law and regulation.
Related: how to assess research evidence →