What is a clinical trial?

Clinical trials (also known as clinical studies) explore whether a new or known drug is safe and effective for humans. They are designed to help us find:

Potential new treatments

Improved versions of treatments already being used

New uses for existing treatments

The participants’ safety is the top priority in every trial. In fact, governments have strict rules to protect the safety and privacy of trial volunteers. All known risks and benefits are outlined in a clinical trial's Informed Consent form, which participants must read and sign before they take part. Typically, trials are split into phases with a larger number of participants each time, giving medical experts more and more data on the safety and effectiveness of a treatment.

Clinical Trial phases

Before it can become available to patients, investigational medicine needs to go through three clinical trial phases before being approved.

It may then go through one further phase to review its long-term effects. Each phase has a different goal and separate hurdles to clear before a potential new treatment can be taken forward.


Small cohort - usually 20-80 participants

The aim is to see how safe the medicine is

Participants are usually healthy volunteers


This phases uses a much larger sample - into the thousands

This gives scientists more data on the safety and effectiveness of the medication being trialled

A drug that successfully passes Phase 3 is likely to be approved by the authorities and ready to be prescribed to the wider public.


Typically between 100-600 participants

The efficacy and safety of the trial medicine is investigated further

The trial may last longer with patients taking several doses


Testing size can vary widely - from less than 100 to more than 1000.

Medication can only progress to Phase 4 trial after it's been approved

This is a more thorough test, examining the longer term safety and efficacy of the treatment and its effects on different types of patient.


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