The drug blocks the effect of the hormone oestrogen
Tamoxifen reduces the risk of breast cancer long after women stop taking the drug, research suggests.
The drug is taken by cancer patients, usually for five years, to prevent the disease returning.
But two studies suggest it might also offer long-term protection for healthy women at high risk of disease.
One study found the drug cut the risk of hormone-sensitive breast cancer by 39% over 20 years, and a second by 34% eight years after therapy was stopped.
A total of 7,145 women at high risk of breast cancer took part in the the International Breast Cancer Intervention Study (IBIS), which is published in the Journal of the National Cancer Institute.
They either took a daily dose of tamoxifen or a dummy drug for five years.
Eight years further on, 87 women who took tamoxifen were diagnosed with hormone-sensitive breast cancer, compared with 129 in the placebo group.
Tamoxifen, which blocks the effect of the hormone oestrogen, has been linked to an increased risk of blood clots.
But the IBIS study found the risk of side effects did not continue after treatment was stopped.
In the second study a team from the Royal Marsden Hospital in London, analysed 20-year data on 2,471 healthy women at high risk of breast cancer who took tamoxifen for about six or seven years.
Professor Tony Howell, from the South Manchester University Hospitals Trust, worked on the IBIS study.
He said: "Previous studies have already shown that tamoxifen lowers the risk of developing breast cancer during active preventive treatment but this is the first time that clear evidence is available on the benefits and side-effects of tamoxifen after treatment with the drug has stopped."
Professor Trevor Powles, lead researcher on the Marsden team, said: "We feel these results are very important because it is first clear evidence that blocking oestrogen can, in the long term, prevent breast cancer in healthy women."
Dr Sarah Cant, of the charity Breakthrough Breast, said: "We know from talking to women at high risk of developing breast cancer that they want to do all they can to reduce their chance of developing this disease and increasing the preventative options available would be extremely welcome.
"However, there are risks and benefits to all preventative treatment options and tamoxifen is not currently licensed for this use in the UK."
Research published this month suggests that many more lives could be saved if women with hormone-sensitive breast cancer were initially given tamoxifen, and then switched to another drug, exemestane, after two to three years.
Breast cancer is by far the most common cancer for women with more than 44,091 new cases in the UK in 2003.