ICTM logo
What is a clinical trial?

Clinical trials are research studies involving patients, which compare a new or different type of treatment with the best treatment currently available (if there is one). Some clinical trials look at possible ways to prevent illnesses, for example by testing new vaccines.

No matter how promising a new drug or treatment may appear during tests in a laboratory, it must go through clinical trials before its benefits and risks can really be known. Trials aim to find out if treatments are safe (what side effects they have), and if they work better than the treatment used currently.

The MRC Clinical Trials Unit carries out clinical trials to look at the best ways to:

  • Prevent illnesses, such as HIV/AIDS
  • Diagnose illnesses – for example by using scans and blood tests
  • Treat illnesses – for example by testing the effectiveness and safety of new drugs or combinations of drugs

Why do we need clinical trials?

Clinical trials are the most reliable and best way of testing treatments. A new treatment is not always better, and can sometimes be worse than existing treatments. Trials are therefore very important to find out whether one treatment is safer and more effective than another, or sometimes to find out if it is just as good.

Different phases of a clinical trial

New treatments usually have to go through a series of clinical trial phases to test whether they are safe and effective. New drugs go through a number of different phases of a trial:

  • Phase I trials aim to test the safety of a new treatment. They look at side effects of a treatment – for example, does it make people sick, raise their blood pressure and so on. Phase I trials involve only a small number of people, who are often healthy volunteers.
  • Phase II trials test the new treatment in a larger group of people who usually have the condition for which the treatment is to be used, to see whether the treatment is safe and has some effect on that condition. Usually less than one hundred people are involved at this stage.
  • Phase III trials involve larger numbers of patients (hundreds or sometimes thousands), who are usually randomised to receive the new treatment or the best available current treatment (or sometimes a placebo). They aim to assess how well the new treatment works, and take longer than Phase I or II trials to run.
  • Phase IV trials are done after the drug has already been approved by drug regulatory authorities. They are carried out to gather information on the drug's effect in various populations and any side effects associated with long-term use.

At the MRC Clinical Trials Unit, most of the trials we run are phase III randomised controlled trials.



For more information about clinical trials, try our section on What is a randomised controlled trial?

Creative Commons Licence
This work is licensed under a Creative Commons Attribution 4.0 International License.