Breast cancer drug tamoxifen, designed to cut recurrence in women with estrogen-receptor positive disease, has been shown to continue working long after women stop taking the drug. And two studies suggest it might also offer long-term protection for healthy women with high risk of developing breast cancer.

One such study found the drug decreases risk of hormone-sensitive breast cancer by 39 percent over 20 years. Another shows a 34 percent decrease for up to eight years after the therapy concludes.

Published in the Journal of the National Cancer Institute, one study -- the International Breast Cancer Intervention Study, or IBIS -- looked at 7,145 women at high risk of breast cancer. And for the first time, clear evidence has surfaced in support of the merits of tamoxifen after the completion of treatment.

IBIS study participants took either a daily dose of tamoxifen or a placebo for five years. At the eight-year mark, 87 women who took the actual drug were diagnosed with estrogen-receptor positive breast cancer. And 129 women in the placebo group were diagnosed with the same disease.

In the second study, researchers from the Royal Marsden Hospital in London investigated 20-year data on 2,471 healthy women at high risk of breast cancer who took tamoxifen for six or seven years. Similar results were found.

Despite the benefits of tamoxifen as a preventative treatment, the drug is not currently approved for this use in the UK, where breast cancer is the most common form of female cancer.