Trials of investigational drugs and biologics are usually described by phase. Each phase answers a different question, and a product generally moves to the next phase only if earlier results support it. The figures below come from the FDA's overview of clinical research (FDA: Step 3, Clinical Research).
| Phase | Typical participants | Typical length | Main purpose |
|---|---|---|---|
| Phase 1 | 20 to 100 healthy volunteers or people with the disease or condition | Several months | Safety and dosage |
| Phase 2 | Up to several hundred people with the disease or condition | Several months to 2 years | Efficacy and side effects |
| Phase 3 | 300 to 3,000 volunteers who have the disease or condition | 1 to 4 years | Efficacy and monitoring of adverse reactions |
| Phase 4 | Several thousand volunteers who have the disease or condition | Varies | Safety and efficacy after approval |
Phase 1: safety and dosage
Phase 1 studies are often the first time an investigational product is given to people. Researchers look at safety, side effects and how the body absorbs and clears it, to find a dose range for later studies. Many enroll healthy volunteers and may involve overnight stays at a research unit. The FDA reports that about 70% of drugs move on from Phase 1.
Phase 2: does it work, and at what dose?
Phase 2 studies enroll people with the condition and look for early evidence that the product works, while continuing to collect safety data. Many are randomized and compare the investigational product with a placebo. The FDA reports that about 33% of drugs move on from Phase 2.
Phase 3: larger confirmation studies
Phase 3 studies are larger and longer. They compare the investigational product with a placebo or the current standard treatment and monitor less common side effects. Their results are a main part of the evidence regulators review. The FDA reports that about 25 to 30% of drugs move on from Phase 3.
Phase 4: after approval
Phase 4 studies take place after a product is on the market. They gather more information on safety and effectiveness in larger and more varied groups of people over a longer time.
Early Phase 1 and "Not applicable"
ClinicalTrials.gov also uses two other labels (ClinicalTrials.gov glossary):
- Early Phase 1 (sometimes called Phase 0) studies give a very small dose to a small number of people for a short time, mainly to learn how the product behaves in the body. They are described in FDA guidance on exploratory studies (FDA: Exploratory IND Studies).
- Not applicable is used for trials without FDA-defined phases, such as many device, behavioral or diet studies. Observational studies have no phase.
Combined phases
Some listings show two phases, such as Phase 1/2 or Phase 2/3. These studies combine the goals of both phases in one protocol, often moving into the second part once the first has answered its questions.
For the bigger picture of how studies are run and overseen, read how clinical trials work.
Source: ClinicalTrials.gov, data processed . Recruiting US sites filtered by distance and site status; summaries shortened.