Selective Estrogen Receptor Modulator
Quick Definition
A selective estrogen receptor modulator (SERM) is a medication that selectively activates or blocks the estrogen receptor depending on the tissue. SERMs include tamoxifen (breast cancer), raloxifene (osteoporosis), and clomiphene/enclomiphene (ovulation induction in women, secondary hypogonadism in men).
In Depth
Selective Estrogen Receptor Modulators (SERMs) bind to the estrogen receptor and produce tissue-specific effects — acting as estrogen agonists in some tissues and antagonists in others. This selectivity makes the class clinically useful for conditions where estrogen activity is desired in one tissue but undesired in another.
Common SERMs and their primary indications:
- Tamoxifen: Primary use in estrogen receptor-positive breast cancer treatment and prevention. Acts as antagonist in breast tissue, partial agonist in bone and uterus. - Raloxifene (Evista): Approved for postmenopausal osteoporosis. Bone agonist, breast antagonist. - Clomiphene citrate (Clomid): Approved for ovulation induction in women. Acts as estrogen receptor antagonist at the hypothalamus, increasing GnRH and gonadotropin release. - Enclomiphene: The trans-isomer of clomiphene. Studied for FDA approval as treatment for secondary hypogonadism in men but does not currently have an approved label.
In men's hormonal health, the SERM most commonly used is clomiphene citrate (or enclomiphene as a single isomer). The mechanism: blocking the estrogen receptor at the hypothalamus and pituitary increases gonadotropin output. The pituitary increases LH and FSH, which signal the testes to produce more testosterone and sperm.
This profile makes SERMs useful for secondary hypogonadism in men who want to preserve fertility — unlike conventional TRT, which suppresses the HPG axis and typically reduces sperm production.
Side effects shared across the SERM class include mood changes, hot flashes, visual disturbances, and increased risk of venous thromboembolism. The visual disturbances (halos, blurring, floaters) are most associated with clomiphene and enclomiphene.
Tamoxifen and raloxifene have established long-term safety records in their FDA-approved populations. Clomiphene and enclomiphene have shorter safety records in male hypogonadism use, which is generally off-label in the U.S. Off-label use should be discussed with your clinician.
Related Terms
Enclomiphene
Enclomiphene is the trans-isomer of clomiphene citrate, a SERM (selective estrogen receptor modulator) used off-label for secondary hypogonadism in men. It raises endogenous testosterone by blocking estrogen feedback at the hypothalamus and pituitary, preserving fertility and testicular function unlike conventional TRT.
Anti-androgen
An anti-androgen is a medication that reduces or blocks androgen activity in the body. Mechanisms include reducing androgen production, blocking the androgen receptor, and inhibiting the conversion of testosterone to DHT. The category includes 5-alpha-reductase inhibitors, androgen receptor blockers, and some other agents.