A Phase I Study:
The initial phase of testing involves 20 to 100 healthy volunteers, who are usually paid for participating in the study.

The study is designed to determine how the drug is absorbed and metabolized in the human body. A phase I study will also observe side effects that occurs as dosage levels are increased.

A Phase III Study:
In a phase III study, a drug is tested in several hundred to several thousand patients. Most phase III studies are randomized and blinded trials.

Phase III studies typically last several years. Once a phase III study is completed, a pharmaceutical company can request FDA approval for marketing the drug.

A Phase II Study:
The second phase tests the medicine's efficacy and usually involves up to several hundred patients.

Most phase II studies are known as Randomized Trials. This is where one group of patients receives the investigational drug, while the other group receives a similar treatment or placebo. Often neither the patients nor the researchers know who is getting the experimental drug and who is not. These are called "blinded" studies.

A Phase IV Studies:
In phase IV studies, they often compare the investigational drug with other drugs already in the market. Also, these studies are often designed to monitor a drug's long-tem effectiveness and impact on a patient's quality of life.