Clinical trials are research studies involving patients, which compare a new or different type of treatment with the best treatment currently available (if there is one). Some clinical trials also look at possible ways to prevent illnesses, for example by testing new vaccines. No matter how promising a new drug or treatment may appear during tests in a laboratory, it must go through clinical trials before its benefits and risks can really be known.

Trials aim to find out if treatments used in health care:

• Are safe
• Have side effects
• Work better than the treatment used currently
• Help people feel better

New treatments usually have to go through a series of trials to test whether they are safe and effective. Trials of new drugs go through a number of phases:

Phase I trials
Phase I trials involve only a small number of people, who may be healthy volunteers. They aim to test the safety of a new treatment and will include looking at any side effects of a treatment, including does it make people sick, raise their blood pressure etc?

Phase II trials
Phase II trials test look to see whether the treatment is effective for the disease for which the treatment is to be used, and at how at safe it is. Phase II trials use the treatment in a larger group of people, usually a few hundred, who will usually have the disease for which the treatment is to be used.

Treatments only move into a phase III clinical trial if phases I and II have been successful.

Phase III trials
Phase III trials test the new treatment in a much larger group of people. They compare the new treatment with the treatment currently in use. Phase III trials look at how well the new treatment works, and at any side effects it may cause. They usually last longer than phase II trials - typically a year or more.

Occasionally the new treatment will be used with a placebo (a dummy drug, which looks like the drug being tested but will have no effect of the disease). This is so the patient will not be aware of what they are taking. Patients are usually randomised to receive the new treatment, the current treatment or a placebo.

Often several thousand patients will be involved in a phase III trial. This is because researchers usually need to be able to measure quite small differences between treatments. The smaller the expected advantage of one treatment over another, the more people will be needed to take part in a trial.

Phase III trials may use different hospitals and patients who live in different countries. It is not uncommon to see phase III trials spread across Europe, USA and parts of Africa.