The regimen is begun with a very low level of the drug and gradually increased as more patients are admitted to the study. Side effects are scrutinized. Patients other than the highest risk are admitted to the study. More people are enrolled. Because more people are enrolled, there is more chance to observe uncommon side effects.
Careful monitoring determines if the therapy is actually working.
During this phase, the treatment used in phase I and II is compared to other treatments. A large number of people are enrolled. Participants are randomly grouped as either the control group or the treatment group. The control group gets the conventional treatments. If there is no other existing treatments, the control group gets a placebo.