Tamoxifen-d5 contains five deuterium atoms at the 3, 3, 4, 4, and 4 positions. It is intended for use as an internal standard for the quantification of tamoxifen (Item No. 13258) by GC- or LC-MS. Tamoxifen is a selective estrogen receptor (ER) modulator, evoking tissue-dependent effects. It is an antagonist of ER action in breast tissue and breast cancer cells and is reported to be effective in the treatment of early breast cancer to prevent tumor growth. Importantly, tamoxifen has been reported to act as an ER agonist in bone and blood vessels, helping to minimize osteoporosis and reduce the risk of cardiovascular disease in post-menopausal women. Also, tamoxifen is a partial ER agonist in uterine tissues and is reported to increase the risk of endometrial carcinoma.