Researchers in Britain say the drug anastrozole can more than halve the development of breast cancer in high-risk women.
A trial on 4000 women, published in the Lancet, showed anastrozole was more effective, cheaper and had fewer side effects than current medications.
It stops the production of the hormone oestrogen, which fuels the growth of the majority of breast cancers.
The BBC reports some countries already offer the drugs tamoxifen and raloxifene to prevent breast cancer. They both block oestrogen activity.
However, they also increase the risk of cancers of the womb, deep vein thrombosis and hot flushes.
Aromatase inhibitors, such as anastrozole, stop oestrogen being produced in the first place and are already used as a treatment for breast cancer.
The BBC reports the study at Queen Mary University of London followed women with a high risk of breast cancer, based on their family history, for an average of five years.
It showed that out of 2000 high-risk women given no treatment there were 85 cases of breast cancer in the study.
But in the same number of women given anastrozole there were 40 cases, with virtually no side-effects.
