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

This trial looked at whether the hormone therapy drug 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 14/04/1992
End 30/03/2001


Phase 3

Summary of results

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

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

In 2014, 22 years after the trial started, the research team looked at how many women in each group had developed breast cancer. They found it was

• 251 out of 3,579 women (7%) in the tamoxifen group
• 350 out of 3,575 women (10%) in the placebo group

The research team worked out that for every 22 women who took tamoxifen, 1 less woman would be diagnosed with breast cancer after 20 years.

They looked at the number of women in each group who died from breast cancer and found it was 31 in the tamoxifen group, and 26 in the placebo group. They also looked at the total number of women who had died from any cause and found it was 182 in the tamoxifen group, and 166 in the placebo group. Although a few more women in the tamoxifen group had died, the difference could be due to chance and it was not statistically significant.
They also looked at how many women developed womb (endometrial) cancer. They found it was 29 women in the tamoxifen group, and 20 in the placebo group. The difference between the two groups was significant only in the 5 years of active treatment.

The number of women who had serious side effects was small in either group. But women in both groups had problems such as hot flushes. 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 concluded that, for women at high risk, taking tamoxifen for 5 years does reduce the risk of developing breast cancer in the next 20 years. They will continue to follow up the women who took part to see if taking tamoxifen has an effect on how long women live.

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: 12 December 2014