Women who take the drug tamoxifen to prevent breast cancer stand a good chance of keeping the disease at bay for 20 years, research has shown.
The protective effect of tamoxifen lasts at least two decades, during which time it reduces breast cancer rates by around 30 per cent, scientists found.
After 20 years, the estimated risk of developing breast cancer was 8 per cent in women treated with tamoxifen for five years compared with 12 per cent for those given a placebo pill.
Professor Jack Cuzick, from Queen Mary University of London, who led the International Breast Cancer Intervention Study (Ibis-I) trial, said: “Tamoxifen is a well-established and effective treatment for certain breast cancers, but we now have evidence of its very long-term preventative benefits.
“The preventative effect of tamoxifen is highly significant, with a reduction in breast cancer rates of around a third, and this impact has remained strong and unabated for 20 years. We hope these results will stimulate more women to consider treatment options for breast cancer prevention if they have a family history of the disease or other major risk factors.”
A total of 7,154 pre- and post-menopausal women aged 35-70 took part in the trial, all of whom were considered at high risk of breast cancer.
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The women were randomly allocated either 20 milligram daily doses of tamoxifen or a placebo for a total of five years. After completing the treatment course, their health was monitored for up to 22 years. The findings, published in The Lancet Oncology journal, showed rates of endometrial cancer were 3.8 times greater among women in the tamoxifen group during the five years of treatment.
Despite helping to prevent breast cancer, there was no evidence that tamoxifen reduced deaths caused by the disease.
In total, 31 women given tamoxifen died from breast cancer compared with 26 of those assigned the placebo.
In addition, five women receiving tamoxifen died from womb cancer compared with none in the placebo group. Prof Cuzick added: “Despite a clear and continuing reduction in breast cancer rates, this has not yet resulted in a reduction in breast cancer deaths. However, the number of deaths is still small compared with the number of breast cancer cases – which is ten times higher. Some of the side effects of tamoxifen are also cause for concern and need continued monitoring, specifically the increased occurrence of endometrial cancer.”
He said that for post-menopausal women an aromatase inhibitor may be a better choice.
The NHS cost effectiveness watchdog, the National Institute for Health and Care Excellence, is now considering calls to include the aromatase inhibitor anastrozole in its guidelines for breast cancer prevention.
For pre-menopausal high-risk women, tamoxifen remained “the only drug of choice for breast cancer prevention”, Prof Cuzick said.
Dr Julie Sharp of Cancer Research UK, which funded the study, said: “The landmark trials show the value of chemo-prevention for women at high risk of breast cancer and highlight just how important these large and long-term studies are.”
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