What is a clinical trial?

A clinical trial is a research investigation when a new treatment is first given to humans, with or without a disease, to establish if the drug is safe and effective at treating the disease.

  • Clinical trials, also known as clinical development or clinical studies, are research investigations that specifically involve human volunteers.
  • They are set up to establish the effectiveness and safety of new treatments including:
    • drugs
    • drug combinations
    • other forms of therapy such as surgery
    • preventative medicines such as vaccines.
  • Clinical trials are only carried out once a treatment has been proved successful in the laboratory using cell or tissue culture systems or in model organisms (Stage 3 of drug development).
  • Carefully regulated clinical trials are currently the most effective way to establish which new treatments are beneficial for human health. 

How does a clinical trial work?

  • Participants in clinical trials are generally divided into two or more groups.
  • Most trials are randomised, which means that the participants are randomly allocated to their treatment groups.
  • Randomisation helps to ensure that people do not know if they are being given the drug under investigation or not, otherwise known as a blinded experiment.
  • In some cases even the investigators (scientists, medical practitioners and so on) do not know who is getting the drug under investigation, until the last moment.
  • This is a double-blinded experiment and ensures that neither patients nor investigators are biasing the outcome of the clinical trial by their prior expectations.
  • Randomisation also helps to minimise the differences between the groups by equally distributing people with particular characteristics. 

The control group

  • One of the groups is always a control group.
  • This is the standard against which the effectiveness of the new treatment is judged.
  • In the control group, the participants may be given no treatment at all, known as placebo, or they may be given the current standard treatment commonly used for the illness under investigation. 

Placebo

  • If the control group is not given an active drug they are often given a placebo.
  • This is usually a treatment that visibly resembles the experimental drug but has no active drug, so will not have an effect on the individual.
  • Placebos are important because they allow clinical trials to be carried out under double-blind conditions.
  • Under double-blind conditions, neither the participants nor the clinical investigators know who is receiving the drug until the trial is complete.
  • This removes any bias and helps to produce fair and objective results. 

Phases in a clinical trial

There are usually four key phases in a clinical trial, but sometimes there can be five:

A flow diagram to show the different phases in a clinical trial.

A flow diagram to show the different phases in a clinical trial.
Image credit: Genome Research Limited

Phase 0

  • Usually trials go straight to Phase I but sometimes a Phase 0 study will be run.
  • Phase 0 involves a very small group of people who are given a very small dose of a drug.
  • Phase 0 is looking to see:
    • if the drug reaches its target, for example, a cancer
    • how the drug behaves in the body
    • how target cells in the body respond to the drug, for example cancer cells.
  • The main aim of Phase 0 is to speed up the development of promising new drugs. Testing some drugs in very small doses in humans gives a clearer idea of drug-target interactions than doing the same in model organisms.

Phase I

  • Phase I studies involve a small group of healthy participants.
  • They are designed to evaluate the safety of the new treatment to see if it causes any side effects.
  • In the case of new drugs, a safe dose range is worked out and any side effects of the drug are identified.
  • This phase usually lasts a few months.

Phase II

  • Phase II involves a larger group of people compared to Phase I. In Phase II these people have the condition being targeted by the therapy.
  • This phase is designed to test how good the new treatment is at treating the disease.
  • To ensure objectivity, these studies are usually randomised and double-blinded.
  • These studies may take up to two years.

Phase III

  • These trials are similar to Phase II trials but on a much larger scale, involving hundreds or even thousands of participants.
  • They collect large amounts of information about the treatment’s safety and effectiveness.
  • Once these trials are completed, the treatment may be approved for general use.
  • This phase may take several years.

Phase IV

  • This phase involves post-marketing trials.
  • It uses feedback from medical practitioners to evaluate the treatment’s long term safety.
  • It is used to find out more about the side effects and safety of the drug.
  • It improves understanding of the long term risks and benefits of the treatment.
  • It shows how well the drug works when it is used more widely.

This page was last updated on 2016-01-25