Tamoxifen is a drug that acts as an estrogen receptor antagonist in breast tissue and is largely used in treatment of breast cancer in post-menopausal women. When ingested, tamoxifen's metabolites bind to estrogen receptors with no effect, thus preventing estrogen from binding to the receptors. Tamoxifen is also used to treat infertility caused by ovulatory dysfunction, primarily in the form of absence of ovulation. Tamoxifen has many other diverse uses, such as (for example) so-called "chemical castration" in convicted sex offenders in the United Kingdom.
Tamoxifen has also been proven to raise testosterone in men by lowering estradiol; estradiol acts as a marker for the body's upper limit production of hormones, including testosterone. Put differently, high amounts of estradiol cause the body to inhibit testosterone production.
Tamoxifen is sometimes indicated in treatment of gynecomastia of disparate causes in: Gynecomastia is "benign proliferation of the glandular tissue of the male breast and results from an imbalance between androgen and estrogen action... Gynecomastia is physiologic in infancy, adolescence and in middle-aged to elderly men and is associated with various conditions such as side effects of medications and disorders with low androgens or high estrogens levels." Eckman and Dobs indicate that tamoxifen is an option to consider in drug-induced gynecomastia, which accounts for 10-25% of all gynecomastia cases.
Adamopoulos et al found that:
Treatment with tamoxifen citrate and testosterone undecanoate improved sperm variables and led to a higher incidence of pregnancy in couples with subfertility related to idiopathic oligozoospermia. In a professional reply to a study, Adamoupolos also commented that "Responsiveness to tamoxifen citrate and testosterone undecanoate is independent of the severity of idiopathic oligozoospermia."
Comparing treatment benefits and drawbacks of SERMs with androgens, Fitts and Powers write:
Testis dysfunction can weaken bone and reduce muscle mass as well as impair sexual function. Testosterone (T) therapy has useful effects on sex organs, bone, and muscle in T-deficient males, but prostate concerns can preclude T use in some men. Although estrogens or other drugs can protect bone in men, gynecomastia makes estrogens unappealing, and other drugs may also be undesirable in some cases. Selective estrogen receptor modulators (SERMs) inhibit estrogen-evoked sex organ growth but mimic estrogen effects on bone and cholesterol and are advantageous for some women. SERMs may also be useful in men who must avoid androgens.
Tamoxifen has also been used to treat anabolic steroid-induced infertility in men:
Anabolic steroid associated male infertility is a little known but potentially treatable form of drug related infertility. We report on a bodybuilder with a 5-year history of steroid use who was azoospermic. He underwent successful gonadotropin replacement and conception was achieved 3 months after therapy was initiated.
 The BIG 1-98 Collaborative Group. Letrozole Therapy Alone or in Sequence with Tamoxifen in Women with Breast Cancer. N Engl J Med, 361:766 Aug. 20, 2009.
 Steiner AZ, Terplan M, Paulson RJ. Comparison of tamoxifen and clomiphene citrate for ovulation induction: a meta-analysis. Hum. Reprod. 20 (6): 1511–5. 2005.
 Sample, Ian. "Q&A: Chemical castration". Guardian Unlimited. (2007)
 Tsourdi E, Kourtis A, Farmakiotis D, Katsikis I, Salmas M, Panidis D. The effect of selective estrogen receptor modulator administration on the hypothalamic-pituitary-testicular axis in men with idiopathic oligozoospermia. Fertil Steril. 2009 Apr;91(4 Suppl):1427-30.
 "Evaluation and management of gynecomastia" (French).Rev Med Suisse. 2009 Apr 8;5(198):783-7.
 Eckman A, Dobs A. Drug-induced gynecomastia. Expert Opin Drug Saf. 2008 Nov;7(6):691-702.
 Adamopoulos DA, Pappa A, Billa E, Nicopoulou S, Koukkou E, Michopoulos J. Effectiveness of combined tamoxifen citrate and testosterone undecanoate treatment in men with idiopathic oligozoospermia. Fertil Steril. 2003 Oct;80(4):914-20.
Adamopoulos DA, Nicopoulou S, Koukkou E, Pappa A, Billa Responsiveness to tamoxifen citrate and testosterone undecanoate is independent of the severity of idiopathic oligozoospermia. J Androl. 2005 Sep-Oct;26(5):565-6; author reply 566-7.
Fitts JM, Klein RM, Powers CA. Comparison of tamoxifen and testosterone propionate in male rats: differential prevention of orchidectomy effects on sex organs, bone mass, growth, and the growth hormone-IGF-I axis. J Androl. 2004 Jul-Aug;25(4):523-34.
 Turek PJ, Williams RH, Gilbaugh JH 3rd, Lipshultz LI. The reversibility of anabolic steroid-induced azoospermia. J Urol. 1995 May;153(5):1628-30.
*The latter article is intended for educational / informational purposes only. THIS PRODUCT IS INTENDED AS A RESEARCH CHEMICAL ONLY. This designation allows the use of research chemicals strictly for in vitro testing and laboratory experimentation only. Bodily introduction of any kind into humans or animals is strictly forbidden by law.