Tamoxifen — Selective Estrogen Receptor Modulator (SERM) — FDA-approved for breast cancer; used off-label in men for gynecomastia treatment, PCT after AAS cycles, and testosterone optimization

Tamoxifen is a SERM with tissue-selective estrogen receptor agonist/antagonist activity. Its effects depend on target tissue: (1) ER ANTAGONIST in breast tissue — blocks ER-alpha mediated proliferation → cancer treatment and gynecomastia treatment; (2) ER AGONIST in bone — maintains bone mineral density (protective effect); (3) ER AGONIST in liver — maintains cardioprotective lipid effects (raises SHBG, lowers LDL); (4) Hypothalamus/pituitary: tamoxifen blocks ER negative feedback at the pituitary → increases LH and FSH secretion → stimulates testicular testosterone production. Male applications: (a) Gynecomastia: blocks breast ER directly — most effective treatment for pubertal and pharmacological gynecomastia; (b) PCT: increases LH/FSH → HPG axis recovery after AAS suppression; (c) Testosterone optimization: mild testosterone-increasing effect comparable to clomiphene. KEY PHARMACOLOGY: tamoxifen is a prodrug; its primary active metabolite endoxifen (formed by CYP2D6) is 100x more potent than tamoxifen itself. CYP2D6 poor metabolizers (7–10% of Caucasians) have dramatically reduced efficacy — significant pharmacogenomic consideration. Active metabolite 4-hydroxytamoxifen (4-OH-T) is also very potent.