Yes. Most breast cancers are ER positive, and clinical trials have tested whether hormone therapy can be used to prevent breast cancer in women who are at increased risk of developing the disease.
A large NCI-sponsored randomized clinical trial called the Breast Cancer Prevention Trial found that tamoxifen, taken for 5 years, reduces the risk of developing invasive breast cancer by about 50% in postmenopausal women who were at increased risk (25). Long-term follow-up of another randomized trial, the International Breast Cancer Intervention Study I, found that 5 years of tamoxifen treatment reduces the incidence of breast cancer for at least 20 years (26). A subsequent large randomized trial, the Study of Tamoxifen and Raloxifene, which was also sponsored by NCI, found that 5 years of raloxifene (a SERM) reduces breast cancer risk in such women by about 38% (27).
As a result of these trials, both tamoxifen and raloxifene have been approved by the FDA to reduce the risk of developing breast cancer in women at high risk of the disease. Tamoxifen is approved for this use regardless of menopausal status. Raloxifene is approved for use only in postmenopausal women.
Two aromatase inhibitors—exemestane and anastrozole—have also been found to reduce the risk of breast cancer in postmenopausal women at increased risk of the disease. After 3 years of follow-up in a randomized trial, women who took exemestane were 65% less likely than those who took a placebo to develop breast cancer (28). After 7 years of follow-up in another randomized trial, women who took anastrozole were 50% less likely than those who took a placebo to develop breast cancer (29). Both exemestane and anastrozole are approved by the FDA for treatment of women with ER-positive breast cancer. Although both are also used for breast cancer prevention, neither is approved for that indication specifically.