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About clinical trials

What are clinical trials?

Clinical trials are research studies that involve people. They test whether new treatments work and how safe these treatments are.

New treatments can include vaccines, medicines, biological therapies, gene therapies, medical devices and surgical treatments. Most treatments in use today were tested in clinical trials.

Why are clinical trials needed?

All new treatments need to be tested in clinical trials. This helps researchers to learn about:

  • How the new treatments work
  • How safe these treatments are
  • What dosage should be used
  • How they compare with existing treatments.

Why do people want to take part in clinical trials?

People take part in clinical trials for many reasons, including:

  • To potentially receive a more effective treatment with fewer side effects
  • To access a potential treatment if there are currently none.

The clinical trial process

Clinical trial phases

The clinical trial process is divided into four main stages, or phases.

Phase 1


Small number of healthy participants.

First test of new treatment in people Treatment is tested for how safe it is and what dose should be used.

Phase 2

Months to Years

Small number of participants with the disease.

Treatment is tested for how it works and how safe it is.

Phase 3


More participants than in phase 2 trials who have the disease.

Treatment is tested for how it works and how safe it is.

Treatment is compared with existing treatments.

Phase 4

Months to Decades

More participants than in phase 3 trials who have the disease

These trials are conducted after the treatment is  approved.

Treatment is tested for
how safe it is.

In the past, clinical trials tested new treatments in a group of individuals who had only one type of health condition. A new type of clinical trial, called a bucket or basket trial, may test a new treatment in individuals who each have different health conditions.

How are clinical trials run?

Clinical trials are run by researchers and a medical team that includes doctors, nurses, pharmacists and other healthcare professionals. The medical team regularly checks the health of trial participants during trial and for some time after the trial is finished.

If participants have serious or unexpected side effects from the trial treatment, the trial doctors will decide if the treatment dosage needs to be changed or if the participants need to stop taking their treatment. Sometimes participants are removed from the trial if their health is getting worse.

By giving some participants a new treatment and other participants a standard treatment, researchers can study how well the new treatment works compared with the standard treatment.

How do researchers know if the treatment has worked?

Many clinical trials are done to find out if a new treatment has clinical benefit. That is, does it improve:

  • The way an individual feels or functions in their daily life
  • How long the person lives (their survival).

Researchers may measure these outcomes in many ways. For example, they may look at a person’s ability to walk or sit without help, or the number of seizures they have each month.

These are checked at the start of the clinical trial, during the trial, and again at the end. This gives researchers a picture of how the participant is benefiting from the treatment, if at all.

Other outcomes that may be measured include:

  • Side effects (treatment-emergent adverse events)
  • Quality of life
  • What happens to the treatment when it is in the body (pharmacokinetics)
  • How the treatment is affecting specific molecules and processes within the body (biomarkers).

Types of clinical trials

All phase 3 clinical trials, and some phase 2 clinical trials, have at least two treatment groups, also known as treatment arms. The first group receives the new treatment being tested. The other group receives a control (or comparator) treatment. People in the control arm may receive either a placebo or an active treatment.

Placebo-controlled trials

A placebo looks like the new treatment but does not contain any of the new treatment. Because of this, it is not expected to have any effect on the participants’ health condition.

Clinical trials that include placebo arms are called placebo-controlled trials. This type of trial gives a very good picture of the effect of the new treatment.

Active-controlled trials

In these trials, participants in the control arm receive the standard treatment for their health condition. This allows researchers to see how safe and effective the new treatment is compared with the standard treatment.

Randomised trials

In these trials, a computer is used to randomly decide which participants will receive the new treatment and which participants will receive a placebo or active comparator.

Blinded trials

In a single-blind trial, the participants are not told whether they are receiving the new treatment or the control.

In a double-blind trial, neither the participants nor the clinical staff treating them know who is receiving the new treatment and who is receiving the control. Only a small number of people running the trial know who is receiving which treatment.

In blinded trials, the new treatment and the control treatment look the same. The trial is ‘unblinded’ once all treatments and follow-ups have been completed. 

Taking part in clinical trials

Potential benefits and risks

Clinical trials have many potential benefits and risks. These need to be carefully considered when deciding whether to join a study.




The aim of each clinical trial is to answer a research question. For example, “Do people taking Treatment X have fewer seizures than those taking Treatment Y”.

Clinical trial participants are carefully chosen to best answer the research question. To do this, each clinical trial has a unique set of inclusion and exclusion criteria.

Inclusion criteria are traits that an individual must have to take part in the trial.

Exclusion criteria are traits that prevent a person from taking part.
Clinical trials may have many different eligibility criteria, such as:

  • Age
  • Gender (male or female)
  • Health condition
  • Symptoms
  • General health
  • Current treatment.

If a person meets all inclusion criteria and none of the exclusion criteria for a clinical trial, they are considered eligible to take part in that trial.

Informed consent

Before taking part in a clinical trial, the participant or their parent or guardian is asked to sign an informed consent form.

The informed consent form contains important information about the trial, such as:

  • The purpose of the trial
  • What is involved
  • How long it will run for
  • Potential benefits and risks.

It is important to read the form carefully. When a participant or their parent or guardian signs the form, they are confirming they have understood all information in the form.

Even if you have signed the informed consent form and joined a clinical trial, you can leave the trial at any time and for any reason!

Key Considerations

It is important to understand the clinical trial that you wish to take part in.

Researchers do many tests on new treatments in laboratories before they test how these treatments work and how safe they are in people during clinical trials. During clinical trials, researchers learn about possible side effects that some people may have with the new treatment. It is important that you understand that the new treatment may or may not help your condition and may or may not cause side effects.

Taking part in a clinical trial is voluntary. You should never feel pressured to take part and you can leave the trial at any time and for any reason.

You should never have to pay for the trial treatment.

Any clinical trial you consider taking part in should be led by healthcare professionals at a clinical site and monitored by an institutional review board.

You can ask the clinical trial staff about their qualifications and experience.

Clinical trial results may be published and made publicly available, but the identity of the participants will always be protected.