Categories, defined by the Food and Drug Administration (FDA), for describing the clinical trial of a drug based on the study’s characteristics, such as the objective and number of participants. There are four phases:

• Phase I trials test an experimental drug, vaccine, or device in a small group of people to evaluate safety, identify side effects and determine safe dosages.
• Phase II trials involve larger groups of people than Phase I and they are designed to assess whether an experimental treatment is safe and whether it works. This phase can last several years.
• Phase III trials are usually large studies comparing the experimental drug or vaccine to a placebo or standard treatment, to evaluate whether the drug works and collect information to allow it to be used safely.
• Phase IV trials are performed once a drug has reached the market, to provide additional information about the best use of the drug. /