A trial looking at stopping tamoxifen after 5 years or continuing treatment to 10 years in women with early breast cancer (aTTom)

Cancer type:

Breast cancer




Phase 3

This analysis compared 5 years and 10 years of tamoxifen treatment for breast cancer.

Cancer Research UK supported this trial.

The aTTom trial has now been changed to aTTom-Extended. In this extended part of the trial the researchers are continuing to collect data on the people who took part to learn more about long term side effects. They are also looking at samples of tissue taken during surgery to learn more about who might benefit from taking tamoxifen for longer than 5 years. 

More about this trial

Many women with early breast cancer take tamoxifen for 5 years because it reduces the risk of the cancer coming back (recurrence).

But researchers wanted to find out if taking tamoxifen for longer was better than taking it for the recommended 5 years.

The women taking part in this trial had breast cancer that was either oestrogen receptor positive, or the oestrogen receptor status wasn’t known.

The aim of the trial was to find out if taking tamoxifen for longer than 5 years would

  • reduce the risk of breast cancer recurrence even more
  • increase side effects

The researchers are continuing to look at data from people who took part in the trial as part of aTTom-Extended. This part of the trial opened in August 2016 and is due to close in August 2020. 

Their aims are to learn more about the benefits of taking tamoxifen, and also monitor the safety of taking this drug for a long time. They will collect the data from either cancer registries or by accessing data through NHS Digital. 

They are also going to look at stored tissue samples to learn more about a test that has been developed to help doctors know who might benefit from long term tamoxifen. 

If you took part in the aTTom trial but do not want the researchers to either continue to collect data about you or use your stored tissue sample for further research you can contact the trial team to let them know. You can email the trial team on: 

Summary of results

Please note the following results are for the aTTom trial. When the aTTom-Extended trial has closed and any results are published we will update this summary. 

The trial team found that taking tamoxifen for 10 years rather than 5 reduced the risk of breast cancer coming back.

This analysis included 6,953 women who had been taking tamoxifen for over 4 years. Half the women stopped taking tamoxifen after 5 years and half carried on taking it.

In 2013, the researchers presented their findings at a large cancer conference. They looked at the number of women who’d had a recurrence of breast cancer and found it was

  • 580 out of 3,468 women who had continued to take tamoxifen
  • 672 out of 3,485 who had stopped taking it after 5 years

They also looked at the number of women who had died and found it was

  • 910 in the group of women who took tamoxifen for 5 years
  • 849 in the group who carried on taking it for longer

One side effect of tamoxifen is an increase in the risk of womb cancer.  The number of women who had developed womb cancer was

  • 102 in the group of women who continued to take tamoxifen
  • 45 in the group who had stopped taking it

But womb cancer can be treated successfully if caught early and the number of deaths from womb cancer was 37 if tamoxifen was taken for 10 years and 20 if taken for just 5 years

The trial team concluded that taking tamoxifen for longer than 5 years reduces the risk of breast cancer coming back and that the benefits outweigh the risks.

We have based this summary on information from the team who ran the trial. The information they sent us has been reviewed by independent specialists (peer reviewed Open a glossary item) but may not have been published in a medical journal.  The figures we quote above were provided by the trial team. We have not analysed the data ourselves.

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

Professor Daniel Rea
Professor Richard Gray

Supported by

Cancer Research UK
Medical Research Council (MRC)
NIHR Clinical Research Network: Cancer
University of Birmingham

Other information

This is Cancer Research UK trial number CRUK/94/001.

Freephone 0808 800 4040

Last review date

CRUK internal database number:

Oracle 27

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

Harriet wanted to try new treatments

A picture of Harriet

“I was keen to go on a clinical trial. I wanted to try new cancer treatments and hopefully help future generations.”

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