Tamoxifen blocks the estrogen receptor in breast cells and in breast cancer cells which express the estrogen receptor. Tamoxifen may be recommended to reduce the risk of developing breast cancer in women who have had prior atypical biopsies. Tamoxifen is also prescribed for women who have had breast cancer that expresses estrogen receptors to decrease recurrence. One study  found that when breast density is carefully measured by computer software, women whose breasts became at least 10% less dense while taking tamoxifen had a 63% reduction in risk of developing breast cancer – whereas those whose breast density did not change did not see a decrease in their risk. Several similar studies [2, 3] in women who have had breast cancer showed that only women whose breast density decreased on tamoxifen had decreased risk of recurrence. Tamoxifen also carries about a 3% risk of blood clots (which could result in pulmonary embolism or stroke) and a smaller risk of endometrial cancer (if the woman still has her uterus).
If a woman is on hormone therapy for menopausal symptoms, her breast density may decrease if she stops taking hormone supplements.
1. Cuzick J. Breast density predicts endocrine treatment outcome in the adjuvant setting. Breast Cancer Res 2012; 14:109
2. Cuzick J, Warwick J, Pinney E, et al. Tamoxifen-induced reduction in mammographic density and breast cancer risk reduction: a nested case-control study. J Natl Cancer Inst 2011; 103:744-752
3. Li J, Humphreys K, Eriksson L, Edgren G, Czene K, Hall P. Mammographic density reduction is a prognostic marker of response to adjuvant tamoxifen therapy in postmenopausal patients with breast cancer. J Clin Oncol 2013; 31:2249-2256