For years the holy grail of breast cancer research has been prevention. The first hint that there could be a meaningful strategy was discovered years ago when women who had undergone oophorectomy (removal of the ovaries) at an early age had a lower incidence of breast cancer later in life.
More recently the drug raloxifene, studied to see whether it could prevent fractures, was found to reduce the incidence of breast cancer by 45%. Then tamoxifen, a drug used to treat breast cancer for over thirty years, was found to reduce the incidence of new breast cancer by 38% on average in several different studies.
The increased risk of uterine cancer and blood clots with tamoxifen impacted upon its widespread acceptance as a prevention measure.