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A trial looking at anastrozole to prevent breast cancer in postmenopausal women (IBIS II Prevention)

This trial looked at whether a drug called anastrozole could reduce the risk of breast cancer in post menopausal women who are at a high risk of getting the disease. The trial was supported by Cancer Research UK.

The female hormone oestrogen can encourage breast cancer cells to grow. Drugs called aromatase inhibitors can stop the body making oestrogen, which may help to prevent breast cancer.

Anastrozole is an aromatase inhibitor that doctors can use as a treatment for breast cancer in women who have been through the menopause (post menopausal women).

Researchers wanted to see if anastrozole could also help to prevent breast cancer in post menopausal women who were at high risk of getting the disease. In this trial, some women took anastrozole, and some took a dummy tablet (a placebo).


Start 02/02/2003
End 31/01/2012


Phase 3

Summary of results

The researchers found that taking anastrozole for 5 years reduced the number of breast cancers in post menopausal women who were at high risk of getting the disease.

The trial recruited nearly 4,000 women who had been through the menopause and were considered to be at high risk of getting breast cancer. This was generally because they had a strong family history of breast or ovarian cancer. Or because they’d had LCIS or DCIS, or certain types of high risk benign breast disease.

This was a randomised trial. The women taking part were put into treatment groups by a computer. Neither they nor their doctor could decide or knew which group they were in. This is called a double blind trial.

  • 1,920 women took anastrozole every day for 5 years
  • 1,944 women took a dummy drug (placebo) every day for 5 years

After monitoring the women for an average of 5 years, the researchers looked at the number of women who had developed breast cancer. They found this was

  • 40 women in the group taking anastrozole
  • 85 women in the group taking the dummy drug

The researchers will follow the women for longer to see if the effect of taking anastrozole lasts and to monitor any side effects

The side effects that women had were similar in both groups. But more women taking anastrozole had problems such as joint pain or stiffness, hot flushes and dry eyes.

The trial team concluded that anastrozole can reduce the risk of breast cancer in post menopausal women who are at high risk of the disease.

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
Professor John Forbes
Professor Anthony Howell

This is Cancer Research UK trial number CRUK/04/032/033.

Supported by

Cancer Research UK
Experimental Cancer Medicine Centre (ECMC)
NIHR Clinical Research Network: Cancer
Queen Mary University of London
Sanofi Aventis
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Updated: 12 December 2013