What are clinical trials?

This page is designed to provide a simple explanation of what clinical trials are and includes some information on terms or phrases you might hear when clinical trials are discussed.

What is a clinical trial?

The simplest definition is that a clinical trial is medical research involving patients.

A clinical trial might be used to;

  • Evaluate a new treatment
  • Evaluate new combinations of existing treatments or change the way in which they are given
  • Compare different ways of diagnosing an illness e.g. scans, blood tests
  • Evaluate techniques for preventing an illness e.g. new vaccine

When setting up a new clinical trial, the overall aim of the trial will be to achieve one or more of the following;
  • Improve the number of people cured (when the illness doesn’t come back)
  • Improve survival (how long people live after treatment)
  • Relieve the symptoms of an illness
  • Reduce the side effects of treatment
  • Improve the quality of life or sense of well-being for people with the illness
Why do we need clinical trials?

Clinical trials are the best way to find out if a new treatment or approach;

  • Is safe and won’t be harmful
  • Has any side effects
  • Is better than the standard treatment used currently
  • Will help people feel better

The treatment offered in a clinical trial is believed to be at least as good as, and possibly better, than the best available treatment.

We need clinical trials to improve treatment and care for patients now and in the future.

Phases of a clinical trial

New treatments are usually tested in a series of steps or phases to see if they are safe and effective;

Laboratory Tests
Phase I Trial - safety and dose
Phase II Trial – safety + effectiveness
Phase III Trial – new vs standard treatment + safety

When a trial or trials have demonstrated that a drug is safe and effective, the manufacturer can apply for the drug to be granted a license, known as a marketing authorisation.

In the UK the Medicines and Healthcare Products Regulatory Agency (MHRA) can grant a license for a drug to be used.

Recruitment and randomisation

These are the processes by which patients are invited to and put onto a clinical trial. Patients will be approached by their Doctor or Nurse and the trial will be explained in detail. Patients will be provided with a patient information sheet and given an appropriate amount of time before deciding whether to take part. The patient information sheet provides details on what the trial is trying to find out, how the patient will be treated and what will be expected of the patient. If the patient decides to take part, then the patient will need to provide consent.

Once consented, the research team will check that the patient meets all the necessary requirements to take part in the trial. These requirements are known as the eligibility criteria and are there to ensure that the trials include the people who might benefit most from a new treatment (e.g. people at a particular stage in their illness) and exclude those for whom the new treatment might not be safe (e.g. pregnant women).

Randomisation is the term given to the random (i.e. based on chance) decision about which treatment the patient receives. Usually there is an equal chance of receiving any of the treatments being compared. By using matched groups, if one group does better than another group, researchers can be sure that this is due to the treatment, as the two groups are similar.

All data about trial participants are kept confidential (patient’s names are removed so individual people will not be identified in the study results). If a participant agrees to join a trial but is not happy about the way it is going, then they can leave the trial at any point and do not need to give a reason

Blind & double blind trials

‘Blind’ is the term given to a trial in which the patient does not know what treatment they will be given.

‘Double-blind’ is the term given to a trial in which neither the patient, doctor nor researchers know what treatment each patient is receiving. In exceptional circumstances, the patient can be ‘unblinded’ and the treatment allocation will become known.

The medicine used will look the same whether it’s new treatment, standard treatment or a placebo. By ‘blinding’ a trial, the researchers are trying to ensure that the chance of bias is reduced (e.g. if the patient knew they were having a new treatment this might make them feel more positive – or more negative – and influence what they report to the researchers).


A placebo is an inactive treatment that’s designed to look exactly like the real one (e.g. in size colour, taste, smell, packaging).

Patients may be given a placebo when there is no standard treatment to compare with the new treatment (e.g. used when a therapy, such as a biological therapy, is being added to the standard treatment to see whether this gives better results).


Whilst the overall aim of carrying out a clinical trial is to improve treatment, the patient’s safety is the primary concern and will never be compromised in order to achieve the trials’ aims.

Trials are heavily regulated and reviewed to ensure that the scientific rationale behind the trial is sound and of potential benefit to the patient. We have summarised the role of a few of the various groups involved in ensuring patient safety;

Independent Scientific Review – The plan for a clinical trial is set out in a ‘protocol’. The protocol is reviewed by a number of independent scientists and experts before the trial is agreed.

Independent Data Monitoring Committees (IDMC) – The committee is set up before a trial begins and as the name suggests, the members are independent of the trial. Their role is to monitor the patient safety and the effectiveness of the treatment during the trial. They are able to access interim (before the end of the trial) results to decide if a clear result has been reached and to ensure that the trial is safe to continue. The committee is able to recommend that a trial is stopped early if there are too many severe side effects or if the results of the new treatment are much better than the standard treatment. The new treatment can then be used instead of the standard treatment so that everyone in the trial can benefit from it.

Trial Steering Committee (TSC) – This committee includes independent members as well as the researchers leading the trial, doctors and nurses. Their role is to ensure that the trial is running well and can advise the researchers on the continuation of it. They may also review any major changes to the protocol and in this case must approve any changes before submission to an ethics committee.

Ethics Committees – No research can be carried out in the NHS without approval from an ethics committee. These are independent committees that protect the rights, safety, dignity and well-being of trial participants. They ensure that patients are fully informed and that they will be compensated if anything goes wrong. The committees will include members of the public, researchers and healthcare professionals.

The committee also need to approve any changes to a trial protocol. Researchers must tell the ethics committee if participants experience unexpected and serious side effects during the trial.

Medicines and Healthcare Products Regulatory Agency (MHRA) – The MHRA are responsible for making sure that trials meet international standards of good practice. Research must report all serious side effects to the MHRA.

National and International Guidelines – There are national and international laws and directives in place to ensure that safety is the primary concern for organisations carrying out research. All organisations that run trials must ensure that they have insurance in place in case people are harmed during a trial.

The Research Governance Framework sets out standards (designing, conducting, recording and reporting of clinical trials) of good Clinical Practice in the conduct of health research.


It can take many years to get the final results. Some trials may need 1000’s of people to take part to show a small but important difference between treatments. If a trial is looking at how long people live after their cancer treatment (overall survival), they need to be monitored for many years.

A final report of a trial may not be published until many years after people were treated. Patients can ask to be informed of the results.

More information

For further information on clinical trials, please follow the links below;

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