a substance (e.g., tamoxifen) that lessens the physiological effects of estrogenic hormones on the tissues that are normally responsive to them. May function by exerting a primarily antagonistic effect on estrogen receptors, lessening the effectiveness, or by blocking estrogen receptor sites. Substances (e.g., raloxifene) that exhibit both agonist and antagonist effect at estrogen receptor sites are called selective estrogen receptor modulators (SERMs). These agents are primarily used in the treatment or prevention of breast cancer, as well as to combat female infertility. Also known as estrogen antagonist.