A trial looking at the drug metformin for early breast cancer

Please note - this trial is no longer recruiting patients. We hope to add results when they are available.

Cancer type:

Breast cancer




Phase 3

This trial is testing the diabetes Open a glossary item drug metformin for early breast cancer. This trial is supported by Cancer Research UK.  

Metformin works by lowering blood levels of a hormone called insulin Open a glossary item. We know from research that people with breast cancer who have low insulin may do better than those with high levels of insulin. We also know from laboratory studies Open a glossary item that metformin may slow the growth of some cancers, including breast cancer.

The aim of this trial is to find out if adding metformin to standard treatment can help people with breast cancer live longer.

Who can enter

You may be able to enter this trial if you are female and

You cannot enter this trial if

  • Your breast cancer has spread to another part of your body
  • You have breast cancer that has come back in the same area (local recurrence)
  • You had cancer in more than 3 lymph nodes
  • You have had chemotherapy in the past month
  • You have had another cancer apart from non melanoma skin cancer or in situ carcinoma of the cervix that has been successfully treated or any other cancer that has been successfully treated and there has been no sign of it for the past 5 years
  • You have diabetes
  • You can’t have metformin
  • You are already taking metformin, sulfonylureas, thiazolidenediones or insulin
  • You have any other medical condition that could affect you taking part in this trial

Trial design

This is an international phase 3 trial. It will recruit 3,580 women from different countries around the world. This is a randomised trial. The people taking part are put into 1 of 2 treatment groups by a computer. Neither you nor your doctor will be able to decide which group you are in. And neither of you will know which group you are in either. This is called a double blind trial.

The women in group 1 will have metformin for 5 years. The women in group 2 will have a dummy drug (placebo) for 5 years. Both metformin and the dummy drug are tablets. You take 1 tablet daily for 4 weeks. You then take 2 tablets a day, 1 in the morning and 1 tablet in the evening. You take your tablets with food.

Randomisation diagram for metformin trial

If you agree to take part in this trial you will have 3 extra blood tests to measure how much sugar and insulin is in your blood (a fasting blood test). Before these blood tests you must not eat or drink anything other than water for 12 hours.

If you agree to take part in this study, the researchers will ask for a sample of tissue taken from when you had surgery to remove your cancer. If you don’t want to give tissue samples for this study, you don’t have to. You can still take part in the trial.

Hospital visits

You see the doctor before taking part in this trial and have some tests. These tests may include

During treatment you see the doctor after 6 months, 1 year and then once a year after that. You have a physical examination and blood tests at each appointment. You have a fasting blood test at 6 months and 5 years. You have a mammogram every year.  

Side effects

The most common side effects of metformin are

  • Diarrhoea
  • Feeling or being sick
  • Tummy (abdominal) bloating
  • Wind (flatulence)
  • Loss of appetite

Recruitment start:

Recruitment end:

How to join a clinical trial

Please note: In order to join a trial you will need to discuss it with your doctor, unless otherwise specified.

Please note - unless we state otherwise in the summary, you need to talk to your doctor about joining a trial.

Chief Investigator

Prof Alastair Thompson (in the UK)

Supported by

Cancer Research UK
Experimental Cancer Medicine Centre (ECMC)
Institute of Cancer Research (ICR)
National Institute for Health Research Cancer Research Network (NCRN)

Other information

This is Cancer Research UK trial number CRUK/11/034.

If you have questions about the trial please contact our cancer information nurses

Freephone 0808 800 4040

Last review date

CRUK internal database number:


Please note - unless we state otherwise in the summary, you need to talk to your doctor about joining a trial.

Deborah wanted to help other breast cancer patients in the future

A picture of Deborah

“Deborah agreed to take part in a trial as she was keen to help other cancer patients in the future. "If taking part in a trial means others might be helped then I’m very happy with that."

Last reviewed:

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