How do clinical trials work?

Find out about the four different phases of clinical trials.

Phase I

The aim of a phase I trial is to find a safe dose of the treatment and look at the possible side effects. If the treatment is safe it will go into phase II to see if it has a positive effect on the patient.

In many cases, this will be the first time the treatment has been tested outside the laboratory. A small number of people – around 30 or fewer – who have no standard treatment options available, are chosen to participate.

Trials in this phase are often known as dose escalation studies. This means that at the start of the trial the first few participants will receive a very low dose of the drug. As more information about the safety of the drug becomes known the next group of patients will receive slightly higher doses. Researchers will monitor the wellbeing of the patients to determine the correct dose of the drug.

Some people in phase I studies will benefit from the new drug but many won't.

If you’re interested in taking part in a clinical trial, you can ask your clinical team if there are any phase I trials running that you may be eligible for.

Specialist units involved in phase I clinical trials:



Cancer Research UK Clinical Trials Unit (CRCTU), University of Birmingham, Birmingham, BS15 2TT

[email protected]


Bristol Clinical Trials Unit, Level 8, Bristol Haematology and Oncology Centre, University Hospital Bristol, Marlborough St, Bristol BS1 3NU


CRUK Cambridge Early Phase Clinical Trials Team, University of Cambridge, Li Ka Shing Centre, Robinson Way, Cambridge, CB2 0RE

[email protected]


NIHR Leeds Clinical Research Facility, Bexley Wing, St. James Hospital, Leeds, LS9 7TF

[email protected]


Royal Liverpool University Hospital Clinical Research Unit, 4th Floor, Linda McCartney Centre, Royal Liverpool University Hospital, Liverpool, L7 8XP

[email protected]


MRC Clinical Trials Unit at UCL, 90 High Holborn, London, WC1V 6LJ

[email protected]


National Institute for Health Research (NIHR) Manchester Clinical Research Facility (CRF), The Christie NHS Foundation Trust, Research Office, 1st Floor, Nowgen Building, 29 Grafton Street, Manchester, M13 9WU 

[email protected]


Sir Bobby Robson Cancer Trials Research Centre, Northern Centre for cancer care, Freeman hospital, Newcastle upon Tyne, NE7 7DN,

[email protected]


Oxford Early Phase Clinical Trials Unit, Churchill hospital, Old Rd, Headington, Oxford, OX3 7LE, 

[email protected]


NIHR Southampton Clinical Research Facility, Southampton Centre for Biomedical Research, C Level West Wing, Mailpoint 218, Southampton General Hospital, Tremona Road, Southampton, SO16 6YD

[email protected]


The Oak Foundation Drug Development Unit, 3rd floor, West Wing, Royal Marsden Hospital, Downs Rd, Sutton, SM2 5PT



Wellcome Trust Clinical Research Facility, Western General Hospital, Crewe Road South, Edinburgh, EH4 2XU

[email protected]


Edinburgh Clinical Research Facility. Royal Infirmary Edinburgh, Edinburgh BioQuarter, Edinburgh, EH16 4SA

[email protected]


CaCTUS, Beatson West of Scotland Cancer Centre, 1053 Great Western Road, Glasgow, G12 0YH

[email protected]


CRUK Clinical Trials Unit Glasgow, Level 11, Boyd Orr Building, University of Glasgow, Glasgow, G12 8QQ

[email protected]

Northern Ireland


Northern Ireland Cancer trials Network, East Podium C Floor, Belfast City Hospital, 51 Lisburn Road, Belfast, BT9 7AB

[email protected]



Velindre Cancer Centre Clinical Trials Unit, Velindre Cancer Centre, Velindre Road, Whitchurch, Cardiff, CF14 2TL

Phase II

This builds on the findings from phase I, improving knowledge of the potential side effects of the treatment and the best dose of the treatment to give.

It usually involves up to 100 people and looks at whether the treatment is having a positive effect on the type of cancer being studied. If tumours respond to the treatment (either by slowing down growth or reducing in size), it moves to phase III.

Find a phase II clinical trial

Phase III

This phase recruits the largest number of participants with hundreds or even thousands of people taking part, often from hospitals across the country.

It compares the new treatment with a standard treatment and will look to see if the new treatment improves overall survival or progression free survival (how long someone stays free of cancer).

Find a phase III clinical trial

Phase IV

This happens after a license to use the drug in normal clinical practice has been granted, and involves studies to monitor the medicine on an ongoing basis to see if there are any unexpected side effects, or if it causes problems in certain categories of people.

Find a phase IV clinical trial

Trials covering more than one phase

Occasionally you may see trials written as phase I/II or phase II/III. This means that that trial covers more than one phase. Phase I/II indicates that the trial covers both phase I and phase II.