Long before any drugs become available to test on human beings they pass through a series of tests in the laboratory.

If the initial results of these tests suggest that further investigation is warranted then it becomes necessary to conduct studies on people. The results of these human studies determine if a drug gets a licence and becomes widely available.

Testing of drugs is divided into several phases.

Phase I

Phase I studies are usually conducted on a small number of healthy volunteers. They aim to provide an initial assessment of a drug providing information on its safety. Over half the drugs that are tested in phase one studies go on to phase II studies

Phase II

A larger number of patients are recruited into phase II studies. These studies may take up to several years. The drugs are often assessed by using a placebo (dummy treatment) in one group of patients whilst the others receive the active drug. They also provide additional safety information and under a half of drugs will progress through to Phase III studies.

Phase III

This involves often thousands of patients to confirm the efficacy of a treatment, provide further safety data and allow a company to get regulatory approval for a drug. Again they can last several years. Most drugs that enter this phase will successfully complete testing.