Clomifene or Clomiphene (also known as Clomifert), marketed as Clomid, is a selective estrogen receptor modulator (SERM) that increases production of gonadotropins by inhibiting negative feedback on the hypothalamus. Clomid inhibits estrogen receptors in the hypothalamus, thereby inhibiting negative feedback of estrogen on gonadotropin production. It may also result in direct stimulation of the hypothalamic pituitary adrenal axis. In short, Clomid provides the bodybuilder with a controlled and dependable post cycle treatment specifically in the return of the testicles to normal size and function (it brings your balls back).

Tamoxifen is an antagonist of the estrogen receptor in breast tissue via its active metabolite, Hydroxytamoxifen. In other tissues such as the endometrium, it behaves as an agonist, and thus may be characterized as a mixed agonist/antagonist.

Tamoxifen is the usual endocrine (anti-estrogen) therapy for hormone receptor-positive breast cancer in pre-menopausal women, and is also a standard in post-menopausal women although aromatase inhibitors are also frequently used in that setting. However, Tamoxifen is popular with anabolic steroid users to reduce the build up of breast tissue (gynecomastia) caused by the excess aromatization of testosterone to estrogen.