Once a new medical treatment has shown promise in laboratory tests, it has to pass three key stages of clinical trials before it can be officially approved for use.

The three research trial stages are:

Phase I trials

Phase I trials focus on the safety of a new medical treatment. They involve a small number of people, who are usually healthy volunteers. The trials look at several measures of safety, such as the side-effects of the treatment.

Phase II trials

Phase II trials focus on whether the treatment is effective. They generally involve a larger group of participants than phase I trials. The participants are usually patients who have the condition the treatment is intended for. Phase II trials also look at safety, but also at dosage levels and efficacy.

Phase III trials

Phase III trials focus on gathering evidence about the specific benefits of the new treatment by comparing it with a similar treatment or a placebo (a ‘dummy’ treatment). Phase III trials last longer than phase I or II trials (usually a year or more) and involve more people – often several thousand treated across different hospitals in different countries.

Once all of these three stages are complete, the new medicine needs to be granted a licence before it can be used to treat people. Medicines and Healthcare Products Regulatory Agency (MHRA) and the European Medicines Agency (EMA) are responsible for granting licences for new medical treatment. Even once a treatment is licensed it will continue to be to be monitored for safety and effectiveness.

You can find more information about the terms used in clinical trials in our research glossary.