Science and Education Publishing Journal of Cancer Research and Treatment 2374-2003 4 3 2016 6 4 Tamoxifen-resistant Breast Cancer: Causes of resistance and Possible Management 37 40 EN Ahmed M. Kabel Department of Clinical Pharmacy, College of Pharmacy, Taif University, Taif, KSA Dania Altalhi Hanan Alsharabi Ola Qadi Maram Ad khan JCRT2016431 10.12691/jcrt-4-3-1 2016 2 26 2016 4 8 2016 6 2 Tamoxifen has been used for the systemic treatment of patients with breast cancer by block the action of estrogen is also used to lower a woman's chance of developing breast cancer if she has a high risk . Treatment success is primarily dependent on the presence of the estrogen receptor (ER) in the breast carcinoma. While about half of patients with advanced ER-positive disease immediately fail to respond to tamoxifen, in the responding patients the disease ultimately progresses to a resistant phenotype. The possible causes for intrinsic and acquired resistance have been attributed to the pharmacology of tamoxifen, alterations in the structure and function of the ER and the interactions with the tumor environment and genetic alterations in the tumor cells.