Tamoxifen citrate is classified as a selective estrogen receptor modulator, with both agonist and antagonist properties (also known as an estrogen agonist/antagonist). It has the chemical designation (Z)2-[4-(1,2-diphenyl-1-butenyl) phenoxy]-N, N-dimethylethanamine 2-hydroxy-1,2,3-propanetricarboxylate (1:1). The primary worry among the athletic/bodybuilding population is gynecomastia, or the very unsightly development of female breast tissue in men. This can be first noticed by the appearance of swelling or a small lump under the nipple. If left to progress, this can develop into a large hard-tissue gynecomastia that may be an irreversible occurrence without surgery. The estrogen can also lead to an increase in the level of water retained in the body, resulting in a notable loss of definition as the muscles begin to look smooth (even bloated) due to the retention of the subcutaneous fluid. Because tamoxifen is a Slective Estrogen-Receptor Modulator (SERM) and possesses both agonist and antagonist properties, it may act as an estrogen in some tissues while blocking the action of estrogen in others. Tamoxifen also possesses the ability to increase production of follicle stimulating hormone (FSH) and luteinizing hormone (LH) which can elevate testosterone production.