Clinical trials that test medications are conducted in phases.
Phase I determines safe dosages of a new drug in a small human population.
Phase II attempts to find out how well people respond to certain treatments. If enough people respond positively, the study may go on to the next level.
Phase III enrolls a large number of people (sometimes thousands worldwide) to test drug efficacy and safety.
Phase IV research requires that a control group receive standard therapy while another group receives the new drug. There also may be other types of Phase IV safety studies.