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

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. 

About this trial
About 4,000 women took part in this trial. They:

• had been through the menopause
• were at high risk of getting breast cancer

This was generally because they had a strong family history of breast 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

Results
The researchers looked at how well treatment worked. 

They looked at the number of women who developed breast cancer in the first 5 years. This was:

• 35 women who took anastrozole
• 89 women who took the dummy drug

The trial team then looked at how well treatment had worked in the longer term. Including 7 years after these women had stopped treatment. The trial team recorded information about the women’s health for 12 years. They followed them up for an average of 10 years. They looked at the number of women who had developed breast cancer. They found this was:

• 85 women who took anastrozole
• 165 women who took the dummy drug

The trial team say this shows that having anastrozole reduced breast cancer developing in just under 5 out of every 10 women (49%) compared to the dummy drug. This reduction was largest in the first 5 years. But continued for 7 years after stopping treatment. 

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.

Over the 12 year follow up, the women didn’t report any new or problematic long term side effects of anastrozole. 

Conclusion
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. In the long term, anastrozole continued to reduce the risk, even 7 years after stopping it. 

The National Institute for Health and Care Excellence (NICE) recommend anastrozole as a treatment option for women at high risk of breast cancer. 

Follow up as part of this trial is now complete. The trial team will collect information from national registries to check the longer term outcomes of women who took part.

Where this information comes from 
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.