Welcome to this site. This web site, cancer.gov, says it can be reduced up to 69%:
http://www.cancer.gov/cancertopics/factsheet/Therapy/
tamoxifen#q12
Can tamoxifen prevent breast cancer?
Research has shown that when tamoxifen is used as adjuvant therapy for early-stage breast cancer, it reduces the chance that the original breast cancer will come back in the same breast or elsewhere. It also reduces the risk of developing new cancers in the other breast. Based on these findings, the NCI funded the BCPT to determine whether taking tamoxifen for at least 5 years can prevent breast cancer in women who have never been diagnosed with breast cancer but who are at increased risk of developing the disease. This study found a reduction in diagnoses of invasive breast cancer among women who took tamoxifen for 5 years. Women who took tamoxifen also had fewer diagnoses of noninvasive breast tumors, such as DCIS or lobular carcinoma in situ (LCIS). After 7 years of follow-up, researchers found similar results. The study found that tamoxifen reduced the occurrence of estrogen receptor-positive tumors by 69 percent, but no difference in the occurrence of estrogen receptor-negative tumors was seen. More information about the BCPT is available on the NCI’s BCPT home page at
http://www.cancer.gov/clinicaltrials/digestpage/BCPT on the Internet.
more posts...