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About clinical trials

About clinical trials

Clinical trials and how they work.

A clinical trial compares the effects of one treatment with another. 

They help doctors understand how to treat a particular illness and may benefit you, or others like you, in the future.

A clinical trial tests if a new treatment is the same, worse, or better than the standard treatment.

The medicines will usually be tested against another treatment called a control. This will either be a dummy treatment (a placebo) or a standard treatment already in use.

Clinical trial phases

All clinical trials of medicines go through a series of phases to test whether they're safe and whether they work.

Phase 1 trials:

  • A small number of people, who may be healthy volunteers, are given the medicine.
  • The drug is being trialled in human volunteers for the first time.
  • Researchers test for side effects and calculate what the right dose might be to use in treatment.
  • Researchers start with small doses and only increase the dose if the volunteers do not experience any side effects, or if they only experience minor side effects.

Phase 2 trials:

  • The new medicine is tested on a larger group of people who are ill. This is to get a better idea of its effects in the short term.

Phase 3 trials:

  • Carried out on medicines that have passed phases 1 and 2.
  • The medicine is tested in larger groups of people who are ill, and compared against an existing treatment or a placebo to see if it's better in practice and if it has important side effects.
  • Trials often last a year or more and involve hundreds, or thousands, of patients.

Phase 4 trials:

  • The safety, side effects and effectiveness of the medicine continue to be studied while it's being used in practice.
  • Not required for every medicine.
  • Only carried out on medicines that have passed all the previous stages and have been given marketing licences – a licence means the medicine is available on prescription. 

Control groups, randomisation and blinding

If you take part in a clinical trial, you'll usually be randomly assigned to either:

  • a treatment group – where you'll be given the treatment being assessed, or 
  • a control group – where you'll be given an existing standard treatment, or a placebo if no proven standard treatment exists.

While the treatments are different in the two groups, researchers try to keep as many of the other conditions the same as possible.

For example, both groups should have people of a similar age, with a similar proportion of men and women, who are in similar overall health.

In most trials, a computer will be used to randomly decide which group each patient will be allocated to.

Many trials are set up so neither the researchers/doctors or trial participants know who's been allocated to receive which treatment. This is called a double blind trial, and it helps reduce the effects of bias when comparing the outcomes of the treatments.


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