High-risk women who take the drug tamoxifen to prevent breast cancer stand a good chance of keeping the disease at bay for 20 years, research shows.
Scientists have found the protective effect of tamoxifen lasts at least two decades, during which time it reduces breast cancer rates by about 30 per cent.
After 20 years, the estimated risk of developing breast cancer was eight per cent in women treated with tamoxifen for five years compared with 12 per cent for women given a placebo.
Professor Jack Cuzick, of Queen Mary, University of London, who led the Ibis-I (International Breast Cancer Intervention Study) trial, said: "We hope these results will stimulate more women, particularly younger women, to consider treatment options for breast cancer prevention if they have a family history of the disease or other major risk factors."
More than 7150 pre- and post-menopausal women aged 35 to 70 took part in the trial, all of whom were considered at high risk of breast cancer.
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 that women on hormone replacement therapy while taking tamoxifen benefited significantly less than those who did not boost their oestrogen levels.
Tamoxifen works by blocking molecular receptors on cancer cells that are stimulated by oestrogen.
It is effective only against hormone-sensitive breast cancer.
Rates of endometrial, or womb, cancer - a known but uncommon side effect of taking the drug - were 3.8 times greater among women in the tamoxifen group during the five years of treatment.
But this increased risk was not seen over the complete follow-up period.
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 to the placebo treatment.
Also, five women receiving tamoxifen died from womb cancer compared with none in the placebo group.
Deaths from other causes were very similar in both groups.
Prof Cuzick said: "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 10 times higher."
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