Most clinical research that involves the testing of a new drug continues in a logical 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. Most clinical trials are classified into one of three phases:

Phase I trials: These initial studies in patients are designed for example to find out if a new drug should be given by mouth, injected into the blood, or injected into the muscle.  The study also evaluates how often the drug should be given and what dose is safe. A phase I trial usually enrolls only a very small number of patients.

Phase II trials: A phase II trial tests the safety of the drug, and starts to find out how well the new drug works.

Phase III trials: These studies test a new drug or a new combination of drugs in comparison to current standard treatment. A patient will usually be randomly assigned to a standard treatment group or a new group receiving the study drug. Phase III trials usually enroll large numbers of people and may be conducted at many medical facilities worldwide.

Furthermore, after a new drug has been approved by the FDA and is available to the public by prescription, the company may continue to  study it in a phase IV trial. The phase IV trial continues to evaluate the side effects, risks, and benefits of the drug in thousands of people and over a lengthy period of time.