All clinical trials of new medicines go through a series of phases to test whether the medicines are safe and whether they work.
The medicines will usually be tested against another treatment, called a control. This will either be a substance containing no medication (a placebo) or a standard treatment that is already in use.
Early research may involve volunteers (who may or may not have a health problem) attending a clinic to assess the effects and safety of a new treatment. Students are often thought of as the typical participants in this type of research, but in fact all kinds of people do it.
You may see advertisements looking for volunteers in newspapers. There may be a payment for this kind of research and usually your expenses will be paid.
From these beginnings, clinical trials become larger and more complicated as the tests of safety and effectiveness become more strictly regulated.
Phase one trials
Phase one trials aim to test the safety of a new medicine.
A small number of people, who may be healthy volunteers, are given the medicine and researchers test for side effects and calculate what the right dose might be to use in treatment (known as dose-ranging studies).
This will usually be the first time that the medicine has been tried on humans, so there is an unavoidable element of risk. To minimise the risk, researchers start with small doses and only increase the dose if the volunteers do not experience any side effects, or if they only experience minor side effects.
Phase two trials
Phase two trials test the new medicine on a larger group of people who are ill, to get a better idea of whether it works and how well it works in the short-term.
Phase three trials
Phase three trials are only for medicines that have already passed phases one and two. They test medicines in larger groups of people who are ill, and compare a new medicine against an existing treatment or a placebo to see if it works better in practice and if it has important side effects.
Phase three trials often last a year or more and involve several thousand patients.
Phase four trials
Phase four trials take place once new medicines have passed all the previous stages and have been given marketing licences. A marketing licence means the medicine can be made available on prescription. Read more about the licensing of medicines.
In phase four trials, the safety, side effects and effectiveness of the medicine continue to be studied while it is being used in practice. Phase four trials are not required for every medicine.
Similar trials are used to assess the effects, both wanted and unwanted, of other kinds of treatment, including physical therapies, surgery, psychological therapies and ways of organising care.
The website of the Association of the British Pharmaceutical Industry (ABPI) has more information about the development of medicines.