Phase I: Researchers test a new drug or treatment in a small group of people (20-80) for the first time to evaluate safety, determine a safe dosage range, and identify side effects.
Phase II: The studied drug or treatment is given to a larger group of people (100-300) to see if it is effective and to further evaluate its safety.
Phase III: The studied drug or treatment is given to large groups of people (1,000-3,000) to confirm its effectiveness, monitor side effects, compare it to commonly used treatments and collect data that will allow the treatment to be used safely.
Phase IV: After a drug or treatment is on the market, this phase is used to delineate additional information, including its risks, benefits, and optimal use.
Source: ClinicalTrials.gov