What are clinical trials? 

A clinical trial is a type of clinical research that compares one treatment with another. It may involve patients or healthy people, or both.

Small studies produce less reliable results than large ones, so studies often have to be carried out on a large number of people before the results are considered sufficiently reliable.

Why clinical trials are important

Doctors and other healthcare professionals and patients need evidence from clinical trials to know which treatments work best. Without this evidence, there's a risk that people could be given treatments that have no advantage, waste NHS resources, and might even be harmful.

Clinical trials help to find out if:

  • treatments are safe 
  • treatments have any side effects 
  • new treatments are better than the standard available treatments

Many NHS treatments have been tested in clinical trials. But the evidence for some treatments is incomplete. Read more about what we don't know

The NHS aims to inform patients about research relevant to them and offer more patients the opportunity to take part in clinical trials if they want to.

What clinical trials can find out

Clinical trials can help:

  • prevent illnesses by testing a vaccine 
  • detect or diagnose illnesses by testing a scan or blood test 
  • treat illnesses by testing new or existing medicines 
  • find out how best to provide psychological support  
  • find out how people can control their symptoms or improve their quality of life – for example, by testing how a particular diet affects a condition

Trials follow a set of rules, known as a protocol, to ensure they're well designed and as safe as possible, they measure the right things in the right way, and the results are meaningful. A full protocol should be available to anyone who's considering taking part in a trial and wants to see it.

Many clinical trials are designed to show whether new medicines work as expected. These results are sent to the Medicines and Healthcare Products Regulatory Agency (MHRA). The MHRA then decides whether to allow the company making the medicine to market it for a particular use.

Read more about safety and regulation.

Entering a trial

If you're receiving treatment for a medical condition, you may be asked if you would like to be part of a trial. You might be interested in finding out about trials taking place so you can volunteer to join one.

Read more about taking part in clinical trials, including how to join a trial.

Testing Treatments interactive

The Testing Treatments interactive (TTi) website contains video and audio material, and cartoons and games explaining how fair tests are carried out.

If you want to find out more about medical research, you can also download the Testing Treatments book for free from the TTi website.

Page last reviewed: 05/01/2015

Next review due: 05/01/2017