Clinical trials are conducted in phases. The trials at each phase have a different purpose, and they help scientists answer different questions:

• In Phase I trials, researchers test an experimental drug or treatment in a small group of people (20 to 80) for the first time to evaluate its safety, to determine a safe dosage range, and to identify side effects.
• In Phase II trials, the treatment is given to a larger group (100 to 300 people) and tested for its effectiveness and safety.
• In Phase III trials, the treatment is given to large groups (1,000 to 3,000 people) to confirm its effectiveness, to monitor side effects, to compare it to other treatments, and to collect information that will allow the experimental drug or treatment to be used safely.
•  In Phase IV trials, post-marketing studies delineate additional information including the drug's risks, benefits and optimal use.
  
  SOURCE: National Institutes of Health