Clinical Trials for new treatments are conducted in phases, with each phase having a different purpose. Once an experimental drug or treatment has demonstrated potential efficacy in laboratory and animal studies, it can move to the first phase of clinical trials. The trials proceed in succession, and a new drug must pass the first 3 phases of trials before it can be considered for licensing by the Food and Drug Administration. The trials at each phase have a different purpose and help scientists answer different questions:

In Phase I trials , researchers test an experimental drug or treatment in a small group of people (20-80) for the first time to evaluate its safety , determine a safe dosage range, and identify side effects. Participants agreeing to participate in these trials tend to be severely ill and have often exhausted all other treatment alternatives.

In Phase II trials , the experimental study drug or treatment is given to a larger group of people (100-300) to see if it is effective and to further evaluate its safety .

In Phase III trials , the experimental study drug or treatment is generally given to larger groups of people to confirm its effectiveness , monitor side effects , compare it to commonly used treatments, and collect information that will allow the experimental drug or treatment to be used safely . The sample size of the Phase III trials depend in part on the prevalence of the outcome each drug is designed to treat.

In Phase IV trials , post-marketing studies reveal additional information including the drug's risks , benefits , and optimal use based on common prescription practices.

(Source: www.clinicaltrials.gov )