Most clinical research that involves the testing of a new drug progresses in a series of steps called phases. This allows researchers to ask and answer questions in a way that results in reliable information about the drug and protects the patients.

Clinical trials are usually classified into one of four phases:

Phase I - Find the right drug dose.
These studies evaluate how a new drug should be given (by mouth, injected into the blood, or injected into the muscle), how often, and what dose is safe.

Phase II - Find out the efficacy of the drug.
A Phase II trial continues to test the safety of the drug, and begins to evaluate how well the new drug works. These studies usually focus on a particular type of cancer.

Phase III - Find out if the drug saves life.
These studies test a new drug, or a new combination of drugs, in comparison to the current standard. A participant is usually assigned to the standard group or the new group at random. Phase III trials often enroll large numbers of people and may be conducted at many doctors' offices, clinics, and cancer centers nationwide and worldwide.

Phase IV - Find additional side effects (if any).
After a treatment has been approved and is being marketed, it is studied in a Phase IV trial to evaluate side effects that were not apparent in the Phase III trial. Thousands of people are involved in a Phase IV trial.