Definition: Clinical trial phases are the steps a drug goes through in human testing. The FDA describes four: phase 1 looks at safety and dose, phase 2 at whether it works and its side effects, phase 3 at how well it works in more people, and phase 4 at safety after approval.
| Phase | Main purpose | Who and how many | How long | Move to next phase |
|---|---|---|---|---|
| 1 | Safety and dosage | 20 to 100 healthy volunteers or people with the condition | Several months | About 70% |
| 2 | Efficacy and side effects | Up to several hundred people with the condition | Several months to 2 years | About 33% |
| 3 | Efficacy and monitoring of adverse reactions | 300 to 3,000 people with the condition | 1 to 4 years | About 25 to 30% |
| 4 | Safety and efficacy | Several thousand people with the condition | After FDA approval | - |
The NCI says a phase III trial compares a new treatment with a standard treatment, and that treatments usually reach phase III only after meeting the goals of phases I and II. It describes phase IV as studying side effects over time once the drug is on the market, looking for ones not seen earlier; see postmarketing experience.
News about a new compound often comes from early phases with small groups. Knowing the phase tells you how much human data there is behind a claim.
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