Clinical trials are divided into four categories, or phases:
Phase I: Drugs are given for the first time to a small group of people. Phase I trials are not designed to show effectiveness; they are designed to show that the drug is not toxic in humans. Little is known about the drugs in Phase I trials, because this is when they are usually tested in humans for the first time or are evaluations of a new dosage or new mode of administration.
Phase II: Drugs are given to a larger group of participants to further evaluate their safety.
Phase III: Drugs are given to large groups of people to confirm their effectiveness, monitor side effects, and compare them to established FDA-approved treatments.
Phase IV: These studies are usually done after the drug has reached the mainstream patient population and are FDA-approved. Information continues to be collected to determine long-term use effects.