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A trial to see if tamoxifen can prevent breast cancer (IBIS 1)

This trial looked at whether the hormone therapy tamoxifen could reduce the risk of breast cancer developing in women who are at a high risk of getting it. This trial was supported by Cancer Research UK.

Some women have a higher risk of developing breast cancer because they have several close relatives who have been diagnosed with it.

The aim of the trial was to compare tamoxifen, a type of hormone therapy, to a dummy (placebo) tablet to see if it can reduce the risk of developing breast cancer in high risk women.


Start 01/08/2001
End 31/12/2002


Phase 3

Summary of results

This trial showed that tamoxifen does reduce the risk of breast cancer developing in women with a high risk of getting it. And that this effect continues after they stop taking tamoxifen.

This trial recruited 7,154 women from around the world in the 1990s. They all had an increased risk of developing breast cancer because of their family history. Half took tamoxifen and half took a dummy (placebo) tablet every day for 5 years.

In 2007 the research team looked at the number of women who had developed breast cancer in each group. That way, they could tell if taking tamoxifen helped prevent breast cancer developing.

  • 142 out of 3,579 women in the tamoxifen group had developed breast cancer
  • 195 out of 3,575 women in the placebo group had developed breast cancer

The numbers of women who had side effects in either group were small. The women taking tamoxifen did have more side effects than the women taking the placebo. But most of the side effects went away or reduced after they stopped taking tamoxifen.

The research team are continuing to follow up the women who took part.

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) and published in a medical journal. The figures we quote above were provided by the trial team. We have not analysed the data ourselves.

Chief Investigator

Professor Jack Cuzick

This is Cancer Research UK trial number CRUK/04/026. 

Supported by

Cancer Research UK
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Updated: 8 September 2009