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 eight per cent in women treated with tamoxifen for five years compared with 12 per cent for women given an inactive placebo pill.
Prof Jack Cuzick, of Queen Mary, University of London, who led the trial, said: “Tamoxifen is a well-established and effective treatment for certain breast cancers, but we now have evidence of its very long-term preventive benefits.
“The preventive 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, 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.”
A total of 7,154 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. Most had a family history of the disease.
The findings, published in The Lancet Oncology journal, showed that women on hormone replacement therapy (HRT) 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 only effective 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.
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 10 times higher.”
He said that for most post-menopausal women, another type of drug called an aromatase inhibitor may be a better choice since it was more effective than tamoxifen and had fewer side effects.
For younger pre-menopausal high risk women, tamoxifen remained “the only drug of choice for breast cancer prevention”, Prof Cuzick said.
Katherine Woods, senior research communications manager at Breast Cancer Campaign, said the study added valuable knowledge to the long-term preventive benefits of the drug.
She said: “This trial has advanced our understanding in this area, however, there is still so much we don’t know about how chemoprevention drugs work - for instance, we need to know far more about which women are likely to benefit from these treatments, which can have harsh and serious side effects.”