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About clinical trials

Before a treatment is made available to the public, it has to be tested in a large group of people to make sure it’s safe and effective. These tests are called clinical trials. 

Why clinical trials are important 

Clinical trials make sure that a new treatment or drug: 

  • is safe and effective
  • really does work better than a placebo (dummy) treatment
  • doesn’t have any dangerous side-effects.

Drugs are first tested in small numbers of people to make sure that it’s safe, before being tested in hundreds or thousands of patients. It’s vital to test in large numbers as good results in one or two patients doesn’t necessarily mean a therapy is safe and effective for everyone.

All treatments have to go through phases of clinical trials before they’re approved. After this process health regulators and doctors can be confident that a treatment is safe and will help the people who need it. The clinical trial stage comes after a treatment is shown to work in experimental models in the laboratory.

Phases of a clinical trial  

A clinical trial has to go through a number of phases to test if the medicine is safe and effective to use. 

Phase 1 makes sure that the medicine is safe for people to use.

Phase 2 tests if the medicine is effective. It tests if it prevents or treats the condition and the dosage that people need to take. 

Phase 3 is usually a trial that involves a larger number of people. It is used to: 

  • confirm the medicine is safe and effective
  • identify any side effects
  • compare the medicine’s benefits to any risks.

Phase 4 is the last phase after the medicine has been approved and is available on the market. It looks for side effects not detected earlier and assesses how well the treatment works.

Find out more about clinical trials

For up-to-date information on clinical trials in the UK, visit the National Institute for Health’s Be Part of Research website. There is also useful information on NHS.uk

If you’re interested in taking part in an international clinical trial taking place outside the UK, visit the World Health Organisation’s International Clinical Trials Registry Platform.

Page last updated: 29 August 2023

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