Enclomiphene

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Primary Mechanism: Central Estrogen Receptor Antagonism

Enclomiphene is a non-steroidal triphenylethylene with a molecular structure closely related to tamoxifen. Pharmacologically, it behaves as a mixed agonist/antagonist at estrogen receptors — a classical SERM profile — but with tissue-specific effects that differ markedly from tamoxifen, raloxifene, and other SERMs used in oncology and osteoporosis.

At the hypothalamus and anterior pituitary, enclomiphene is a pure antagonist of estrogen receptor alpha (ERα). Under physiological conditions, circulating estradiol (E2) — most of which is produced in men via peripheral aromatization of testosterone in adipose tissue, liver, and skin — binds hypothalamic ERα and provides the major negative feedback signal that throttles GnRH pulse generation. This feedback loop is the biological governor on the HPG axis: as testosterone rises, aromatization generates estradiol, estradiol suppresses GnRH, and LH/FSH output falls, completing the circuit.

When enclomiphene occupies hypothalamic ERα, estradiol can no longer exert this suppressive feedback. The hypothalamus perceives the estrogen signal as absent and responds by:

1. Increasing GnRH pulse frequency: Kisspeptin neurons in the arcuate nucleus and anteroventral periventricular nucleus (AVPV) are derepressed, leading to more frequent GnRH bursts (Dwyer et al., 2015).
2. Amplifying pituitary gonadotroph response: The pituitary becomes more sensitive to GnRH stimulation, producing larger LH and FSH pulses per GnRH input.
3. Driving downstream testicular steroidogenesis: LH stimulates Leydig cells to synthesize testosterone from cholesterol via the classical steroidogenic pathway (StAR → P450scc → 3β-HSD → CYP17 → 17β-HSD). FSH supports Sertoli cell function and spermatogenesis.

The net effect is an upward shift in the entire HPG axis set point — higher LH, higher FSH, and higher endogenous testosterone — without exogenous hormone administration. In clinical studies, typical LH increases range from 2-4x baseline, testosterone rises 50-100% from hypogonadal baseline, and the effects are maintained with chronic dosing because enclomiphene does not accumulate or cause tachyphylaxis (Kaminetsky et al., 2013).

The Enclomiphene vs. Zuclomiphene Distinction

Clomiphene citrate as prescribed for female ovulation induction is a racemic mixture of two stereoisomers: enclomiphene (the E-isomer, ~62%) and zuclomiphene (the Z-isomer, ~38%). These two molecules differ only in the geometric orientation around a single double bond, but their pharmacology is dramatically different:

• Enclomiphene is a short-acting ERα antagonist with a half-life of approximately 10 hours and complete clearance within 1-2 weeks of discontinuation. It does NOT accumulate with chronic dosing.
• Zuclomiphene is a long-acting weak ERα agonist with a half-life of 30+ days and measurable tissue concentrations for months after stopping chronic therapy. It DOES accumulate.

This distinction matters enormously for men. When men take clomiphene long-term (the common "clomid TRT" approach), zuclomiphene gradually accumulates and begins to produce estrogen-agonist effects: visual disturbances (shimmering, floaters, rarely permanent retinal changes), mood destabilization (depression, anxiety, emotional lability), libido suppression paradoxically overriding the testosterone boost, and potentially long-term safety concerns from chronic ERα agonism in a male. Many of the complaints men have about "clomid" — the mood crashes, the vision issues, the libido problems — are attributable to zuclomiphene rather than to the antagonist component.

Purifying enclomiphene removes zuclomiphene entirely. The result is a cleaner molecule that delivers the HPG axis stimulation without the off-target agonist burden. In head-to-head comparisons and clinical trials, enclomiphene produces substantially fewer mood, visual, and libido side effects than equipotent doses of clomiphene (Wiehle et al., 2013). This is the central pharmacological argument for using enclomiphene rather than clomiphene for extended HPG axis support.

Fertility Preservation Pharmacology

The clinical feature that sets enclomiphene apart from exogenous testosterone is its effect on spermatogenesis. Exogenous testosterone (whether injectable, topical, or oral) suppresses LH and FSH via direct pituitary negative feedback after peripheral conversion to estradiol and via direct androgen receptor feedback in the hypothalamus. This suppression drops intratesticular testosterone concentrations from their normal range of 500-1,000 ng/mL down to serum levels (5-10 ng/mL) — a 50-100x decrease. Since spermatogenesis requires extraordinarily high local testosterone concentrations (20-100x serum levels) to proceed through meiosis, the result is progressive oligospermia and, in most men, frank azoospermia within 3-6 months of consistent TRT use.

Enclomiphene has the opposite effect. By stimulating rather than suppressing the HPG axis, it drives LH to support intratesticular testosterone production AND drives FSH to support Sertoli cell function and spermatogenesis directly. In men with baseline sub-fertility from secondary hypogonadism, enclomiphene typically improves sperm concentration, motility, and total motile count over 3-6 months of therapy (Shabsigh et al., 2005 on clomiphene, with analogous data for enclomiphene). For men who want both symptomatic relief from low testosterone AND preservation of reproductive potential, this is a substantial advantage over TRT.

Hepatic Metabolism and Pharmacokinetics

Enclomiphene is orally bioavailable with peak plasma concentrations reached 4-8 hours after dosing. It undergoes extensive first-pass hepatic metabolism primarily via CYP2D6 and CYP3A4, with the main metabolite 4-hydroxy-enclomiphene retaining some antiestrogenic activity. Terminal elimination half-life is approximately 10 hours, which is why once-daily dosing is adequate for stable LH/FSH elevation. Excretion is primarily biliary/fecal, with minor urinary elimination.

Because of CYP2D6 involvement, men who are CYP2D6 poor metabolizers (~7% of Caucasians, ~2% of East Asians) may experience higher plasma concentrations and more pronounced effects at standard doses. CYP2D6 inhibitors (paroxetine, fluoxetine, bupropion) may also elevate enclomiphene levels modestly, though clinically significant interactions are uncommon. CYP3A4 inducers (rifampin, carbamazepine, St. John's wort) may reduce enclomiphene plasma levels and blunt response.

Off-Target Receptor Activity

Relative to tamoxifen and other SERMs, enclomiphene has minimal off-target activity at estrogen receptor beta (ERβ), androgen receptor, progesterone receptor, or glucocorticoid receptor. This relatively clean receptor profile contributes to its favorable tolerability in men, as most SERM-related side effects (hot flashes with tamoxifen, endometrial effects, bone density changes) arise from tissue-specific agonism at ERα in organs outside the HPG axis. Enclomiphene appears to be a near-pure HPG-axis modulator with minimal extra-axis activity at therapeutic doses.

Enclomiphene citrate (ENC, Androxal, trans-clomiphene) is the pure trans-isomer of clomiphene citrate, a non-steroidal selective estrogen receptor modulator (SERM) developed originally by Repros Therapeutics for the treatment of secondary (hypogonadotropic) hypogonadism in men who want to preserve fertility. Where clomiphene citrate — the classical ovulation induction drug used in women since FDA approval in 1967 — is a 62:38 mixture of two geometric isomers (enclomiphene and zuclomiphene), enclomiphene is the short-acting antagonist component that carries virtually all of the therapeutically useful activity at the male hypothalamic-pituitary-gonadal (HPG) axis, while zuclomiphene is a long-half-life weak estrogen agonist that accumulates with chronic dosing and is blamed for much of clomiphene's off-target visual, mood, and gynecomastia-risk profile.

In men, enclomiphene works by competitively blocking estradiol at estrogen receptor alpha (ERα) in the hypothalamus and pituitary. Negative feedback from circulating estradiol normally throttles gonadotropin-releasing hormone (GnRH) pulse generation; when enclomiphene occupies these receptors, the hypothalamus is "blinded" to estrogen, GnRH pulse frequency increases, the pituitary ramps up luteinizing hormone (LH) and follicle-stimulating hormone (FSH) secretion, and the testes respond by increasing endogenous testosterone production and spermatogenesis. The net clinical effect, in men with a functional HPG axis that is simply being under-driven (secondary hypogonadism), is a restoration of serum total testosterone into the mid-normal range — often 500-700 ng/dL from baselines below 300 ng/dL — while preserving or improving sperm count, motility, and morphology. This stands in sharp contrast to exogenous testosterone replacement therapy (TRT), which suppresses LH/FSH, shuts down intratesticular testosterone production, and typically drives sperm counts toward azoospermia within 3-6 months of use (Anawalt, 2020).

Enclomiphene sits in an unusual regulatory limbo. Repros Therapeutics ran two successful Phase 3 trials (ZA-301 and ZA-302) in overweight men with secondary hypogonadism, demonstrating that once-daily 12.5 mg or 25 mg enclomiphene restored morning total testosterone while maintaining or increasing sperm concentration relative to both placebo and topical testosterone gel (Kim et al., 2016). Despite these results, the FDA declined to approve Androxal in 2015, citing concerns about the clinical meaningfulness of the primary endpoints and the absence of long-term cardiovascular outcomes data — concerns that parallel the broader regulatory pushback against all low-T treatment paradigms during the 2013-2016 era. Repros subsequently went bankrupt, and the molecule fell into a commercial gray zone: not FDA-approved, but sold widely through US telehealth clinics and compounding pharmacies under state physician supervision as an "off-label" or "investigational" testosterone tuning tool, particularly for men who want to avoid the fertility penalty of TRT.

For bodybuilders, biohackers, and men pursuing testosterone tuning, enclomiphene has become a central pillar of the "fertility-preserving" and "post-cycle therapy" toolkits. It is used as (1) a monotherapy for men with secondary hypogonadism who want natural production rather than exogenous testosterone; (2) a post-cycle therapy (PCT) agent after anabolic steroid cycles to restart the HPG axis; (3) an adjunct or alternative to hCG for men on TRT who want to preserve testicular volume and fertility; and (4) a mild endogenous testosterone booster in men with borderline hypogonadism or age-related T decline. Its oral dosing, lack of injection requirement, favorable side effect profile relative to clomiphene itself, and fertility-preserving mechanism make it one of the more popular prescription-gray-market molecules in the men's health tuning space.

This entry covers the full pharmacology, clinical evidence base, protocol considerations, and safety profile of enclomiphene citrate. It is intended for educational and research reference only — enclomiphene is a prescription drug in most jurisdictions, is NOT FDA-approved for any indication, and the decision to use it should involve a qualified physician who can order appropriate baseline and follow-up labs (total testosterone, free testosterone, estradiol, LH, FSH, SHBG, CBC, CMP, lipid panel, and semen analysis when fertility is a concern). Cross-reference this page with HCG, Gonadorelin, and Kisspeptin-10 for a complete picture of HPG axis modulation strategies.

IUPAC Name

(E)-2-[4-(2-chloro-1,2-diphenylethenyl)phenoxy]-N,N-diethyl-ethanamine

CAS Number

15690-57-0

Molecular Formula

C26H28ClNO

Molecular Mass

405.96 g/mol

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