The global study was led by University of Melbourne and the Peter MacCallum Cancer Centre and published in the Journal of Clinical Oncology.
The study involved about 2,500 women from Europe, North America and Australia who have inherited mutations in BRCA1 or BRCA2, the breast cancer susceptibility genes, and who had been diagnosed with breast cancer. About one-third of these women were placed on tamoxifen.
Tamoxifen has been used for decades to treat breast cancer and has recently been shown to prevent breast cancers in many women.
Until now, there has been limited information about whether it reduces breast cancer risk for women who are at the very highest level of risk with BRCA1 or BRCA2.
Lead author, Professor Kelly-Anne Phillips says this study, the largest to date, suggests that it could work for these high-risk women by halving their breast cancer risk.
"In the past, the only way of reducing breast cancer risk for these high-risk women was to do invasive surgery to remove their breasts and/or ovaries. For women who choose not to undergo such surgery, or who would prefer to delay surgery until they are older, tamoxifen could now be a viable alternative."
Such was the case for US actress Angelina Jolie who was found to carry a mutation in one of these genes.
Professor John Hopper, co-author from the School of Population and Global Health at the University of Melbourne, says "In light of our findings, it is clear that women who have a mutation in BRCA1 or BRCA2 should review their management plan with their specialist and re-discuss the options available to them to lower that risk."
This important finding has come from more than 20 years of research involving breast cancer families recruited from cancer registries and clinics across the country.
"Without the generous contributions of those families we would not be able to make such discoveries which help future generations fight breast cancer," he says.