Enclomiphene Citrate Phase III Complete

What It Is

Enclomiphene citrate is the trans-(E)-isomer of clomiphene citrate, a triphenylethylene-class selective estrogen receptor modulator (SERM). Standard clomiphene citrate (FDA-approved as Clomid for ovulation induction in women since 1967) is a mixture of two geometric isomers: enclomiphene (the trans-isomer, ~62% of the clinical product) and zuclomiphene (the cis-isomer, ~38%). The two isomers have markedly different pharmacology: enclomiphene acts as a relatively pure estrogen receptor antagonist with a short half-life (~10 hours), while zuclomiphene acts as a partial estrogen agonist with an extremely long half-life (~14 days) that accounts for many of clomiphene's mood, vision, and estrogenic side effects.

Enclomiphene was developed as a single-isomer pharmaceutical product by Repros Therapeutics under the proposed brand name Androxal, specifically targeting secondary hypogonadism in men who wish to restore testosterone while preserving fertility. The clinical thesis: by selectively antagonizing estrogen receptors at the hypothalamus and pituitary, enclomiphene removes the negative-feedback brake on the hypothalamic-pituitary-gonadal (HPG) axis. Gonadotropin-releasing hormone (GnRH) pulse frequency increases, luteinizing hormone (LH) and follicle-stimulating hormone (FSH) rise, and the testes — appropriately stimulated — produce endogenous testosterone with maintained spermatogenesis. This is fundamentally different from exogenous testosterone replacement therapy (TRT), which raises testosterone but suppresses LH/FSH and can impair fertility within months.

Repros conducted multiple Phase 2 and Phase 3 trials of enclomiphene through the 2010s comparing 12.5 mg and 25 mg daily oral doses against placebo and topical testosterone (AndroGel). The trials repeatedly demonstrated that enclomiphene (1) raised serum total and free testosterone into the eugonadal range comparable to topical testosterone, (2) preserved (and in some men improved) sperm count, while (3) AndroGel suppressed LH/FSH and reduced sperm count. Despite these positive efficacy results, the FDA issued a Complete Response Letter to Repros in December 2015, declining to approve Androxal. Specific FDA concerns included questions about the clinical relevance of the testosterone elevation in the obese-hypogonadal target population. The EMA's CHMP refused marketing authorization in January 2018 on similar grounds. Repros formally discontinued enclomiphene development across all indications in April 2021.

That regulatory history — Phase 3 efficacy demonstrated but commercial approval not granted — has shaped the present landscape. Enclomiphene is currently compounded by 503A and 503B pharmacies in the United States as a clinician-prescribed off-label men's health treatment, and is one of the most commonly prescribed compounds in modern men's hormone optimization clinics for men who want testosterone restoration without sacrificing fertility. The compound is on the FDA's bulk-substance compounding lists and is widely available through the men's health telehealth ecosystem. Anyone considering it should understand: efficacy for endogenous testosterone elevation is well-supported by Phase 3 data; the FDA-approval status remains absent; and current use is off-label compounded prescription rather than approved pharmaceutical product.

Mechanism of Action

Enclomiphene's mechanism is a clean SERM-mediated disruption of the estrogen-feedback loop on the HPG axis, with downstream restoration of endogenous testicular testosterone production.

• Estrogen receptor antagonism at hypothalamus/pituitary — Enclomiphene binds and competitively antagonizes estrogen receptors (predominantly ERα) at the hypothalamus and anterior pituitary. In normal physiology, circulating estradiol provides negative feedback to suppress GnRH pulse frequency and LH/FSH secretion. By blocking this estrogen feedback, enclomiphene removes the brake.
• Increased GnRH pulse frequency — With estrogen feedback inhibition, hypothalamic GnRH neurons fire more frequently. Pulse frequency increase is the molecular substrate for LH elevation.
• LH and FSH elevation — Increased GnRH pulse frequency drives anterior pituitary gonadotrope LH and FSH secretion. Both rise dose-dependently with enclomiphene treatment.
• Leydig cell stimulation → endogenous testosterone — Elevated LH stimulates testicular Leydig cells to produce testosterone. The testicular machinery is intact in secondary (hypogonadotropic) hypogonadism — the deficit is in upstream gonadotropin signaling. Enclomiphene corrects the upstream deficit; testicular response is downstream.
• FSH preserves spermatogenesis — Elevated FSH maintains Sertoli cell support of spermatogenesis. This is the key pharmacological feature distinguishing enclomiphene from exogenous TRT, which suppresses both LH and FSH (causing testicular atrophy and impaired spermatogenesis).
• Pure antagonism vs partial agonism (the isomer distinction) — Enclomiphene is a relatively pure estrogen receptor antagonist. Zuclomiphene (the cis-isomer in standard clomiphene) has substantial estrogen partial-agonist activity at peripheral tissues, contributing to the visual, mood, and estrogenic side effects of mixed clomiphene. Single-isomer enclomiphene was developed to avoid these zuclomiphene-mediated effects.
• Tissue-specific effects (SERM character) — Like other SERMs (tamoxifen, raloxifene), enclomiphene's effects vary by tissue. At hypothalamus/pituitary it is an antagonist (driving the desired LH/FSH rise). At bone it has weak estrogen-agonist activity (theoretically protective). At endometrium and breast tissue effects vary; relevant primarily for the female ovulation-induction context.
• Estradiol elevation (downstream) — Elevated testosterone feeds aromatase activity. Serum estradiol typically rises modestly during enclomiphene treatment — the increase is secondary to elevated testosterone and aromatization, not direct enclomiphene effect. Some clinicians monitor and treat with low-dose aromatase inhibitor (anastrozole) if estradiol rises out of optimal range.
• SHBG modulation — Enclomiphene tends to raise sex hormone binding globulin (SHBG) modestly; this can reduce free testosterone fraction even as total testosterone rises. Some users see big total-T gains with smaller-than-expected free-T improvements.
• Half-life and pulse architecture — Enclomiphene's ~10-hour half-life supports once-daily oral dosing. Critically, the absence of long-half-life zuclomiphene means the HPG axis is not chronically suppressed by drug accumulation — a feature, not a bug.
• What it does not do — Does not directly bind testosterone receptors. Does not work in primary hypogonadism (Leydig cell failure — there is no testicular response to amplify). Does not enhance fertility in men with normal HPG function. Does not bypass the testes; if testes are non-functional, enclomiphene will not produce testosterone elevation.

What the Research Shows

Enclomiphene has the most extensive Phase 2/3 human evidence base of any compound on this database that is not FDA-approved. Repros Therapeutics conducted multiple high-quality randomized controlled trials.

• Hill et al. 2009 — Androxal vs AndroGel comparison — 14-day randomized double-blind placebo- and active-controlled trial in hypogonadal men. Doses: 12.5, 25, 50 mg enclomiphene vs placebo vs AndroGel 1%. Enclomiphene increased total testosterone dose-dependently (412 ng/dL at 12.5 mg, 520 at 25 mg, 589 at 50 mg) without disproportionate DHT elevation. Foundational human pharmacodynamic study.
• Wiehle 2013–2014 — PK/PD (PMC4155868) — 48 men randomized to 6.25, 12.5, 25 mg enclomiphene daily vs transdermal testosterone for 6 weeks. Established dose-response for testosterone elevation. Critically, LH and FSH rises persisted for at least 7 days after enclomiphene discontinuation, demonstrating a longer functional duration of effect than the ~10-hour half-life would predict.
• Kim/Wiehle 2016 BJU Int — Phase 3 RCTs (PMID 26496621) — Two parallel randomized double-blind double-dummy placebo-controlled multicenter Phase 3 trials. Compared two enclomiphene doses (12.5 mg and 25 mg) vs AndroGel 1.62% vs placebo in overweight men aged 18-60 with secondary hypogonadism. Findings: enclomiphene consistently increased serum total testosterone, LH, and FSH; restored normal serum testosterone; maintained sperm concentrations in the normal range; AndroGel raised testosterone but reduced sperm counts. This is the pivotal efficacy dataset and the basis for current off-label clinical use.
• IGF-1 lowering signal (Wiehle 2013 AACR abstract) — In secondary hypogonadism trial cohorts, oral enclomiphene lowered serum IGF-1 (more than topical testosterone or placebo) while raising testosterone. Authors hypothesized implications for cancer prevention given the IGF-1 / cancer epidemiologic association — speculative but intriguing.
• Long-term efficacy and safety (multiple Repros studies) — Multi-month extension studies showed sustained testosterone elevation with continued daily dosing. Safety profile compared favorably to topical testosterone.
• Decades of mixed-clomiphene literature — Off-label clomiphene use in men for hypogonadism and idiopathic male infertility has generated substantial supportive literature for the mechanism. Limitations: zuclomiphene-mediated side effects complicate interpretation.
• Helo 2015 J Sex Med — clomiphene vs anastrozole — Randomized prospective double-blind comparison of clomiphene vs anastrozole in raising testosterone in hypogonadal infertile men. Both effective; different mechanisms (SERM vs aromatase inhibitor).
• FDA Complete Response Letter (December 2015) — FDA declined to approve Androxal NDA. Specific concerns included questions about the clinical relevance of testosterone elevation in the obese-hypogonadal target population.
• EMA refusal (January 2018) — CHMP refused marketing authorization on similar grounds.
• Discontinuation of all development (April 2021) — Repros formally ended enclomiphene development across all indications.

Human Data

Enclomiphene has the most extensive completed-human-trial dataset of any compound on this database that lacks FDA approval.

• Hill et al. 2009 — Foundational 14-day Phase 2 dose-finding study. Established dose-response.
• Wiehle et al. 2013–2014 PK/PD (PMC4155868) — 48 men, 6 weeks, three doses of enclomiphene vs transdermal testosterone. Established sustained LH/FSH/T elevation for at least 7 days post-discontinuation.
• Kim/Wiehle et al. 2016 BJU Int Phase 3 (PMID 26496621) — Two parallel randomized double-blind multicenter Phase 3 trials. Pivotal efficacy dataset.
• NCT01406964, NCT01403038 — Repros Phase 3 ZA-203 / ZA-204 — Registered on ClinicalTrials.gov. ~250+ men per trial.
• Long-term extension studies — Multi-month to multi-year extensions demonstrated sustained efficacy and safety.
• Off-label clinical experience (decade+ of compounded use) — Hundreds of thousands of patient-months of off-label compounded enclomiphene use through men's health clinics and telehealth providers in the U.S. since the 2015 FDA decision. Real-world experience strongly supports trial efficacy data.
• Older mixed-clomiphene off-label literature — Decades of clomiphene 12.5–25 mg daily off-label use in male hypogonadism and idiopathic male infertility. Effective for T elevation; visual and mood side effects (largely zuclomiphene-mediated) are the primary tolerability limitation.
• Female ovulation induction (Clomid) — FDA-approved 1967. Decades of safety experience in women. Provides general safety framework for the SERM class.

Dosing from the Literature

Doses below combine Phase 3 trial protocols with current clinical men's health practice. Always work with a licensed clinician.

Reconstitution & Storage

Enclomiphene is a small-molecule oral drug. It is not lyophilized and does not require reconstitution. It is supplied as compounded oral capsules or tablets, typically 12.5 mg or 25 mg strengths.

• No reconstitution required — Oral solid dosage form. Do not attempt to dissolve for injection.
• Bioavailability — Good oral bioavailability. Take with food to minimize GI side effects.
• Tmax — Approximately 4–6 hours post-dose.
• Half-life — ~10 hours for enclomiphene specifically (vs ~14 days for the cis-isomer zuclomiphene). The single-isomer formulation is the key pharmacokinetic advantage.
• No special storage — Room temperature dry; no refrigeration required.

→ Use the Kalios Dosing Calculator for enclomiphene cycle planning

Side Effects & Risks

Enclomiphene has the most well-characterized side-effect profile of any compound on this database that lacks FDA approval, due to multiple completed Phase 3 trials.

• Generally well-tolerated — Single-isomer enclomiphene avoids most of the side effects associated with mixed clomiphene that are attributable to the long-half-life zuclomiphene cis-isomer.
• Mild headache — Most commonly reported; usually transient, often resolves within 1–2 weeks.
• Hot flashes / vasomotor symptoms — Class effect of estrogen receptor antagonism; less prominent than with mixed clomiphene; can be dose-related.
• Mild mood changes — Some users report mild anxiety, irritability, or low mood. Less common with single-isomer enclomiphene than with mixed clomiphene.
• Visual disturbances (rare) — Reported with mixed clomiphene at higher doses; rare with single-isomer enclomiphene at typical 12.5–25 mg doses. If new visual changes occur, stop and seek evaluation.
• SHBG elevation — Modest. Total testosterone rises more than free testosterone in some users due to SHBG increase.
• Estradiol elevation — Secondary to testosterone elevation and aromatization. Some clinicians monitor; low-dose anastrozole if E2 rises out of range.
• GI tolerability — Mild nausea or stomach discomfort; mitigated by dosing with food.
• Theoretical thrombosis concern — SERM class has documented venous thromboembolism risk (predominantly characterized for tamoxifen and raloxifene in women). Risk in men with enclomiphene is presumed lower but not rigorously quantified. Avoid in men with prior VTE history or other thrombotic risk factors.
• Theoretical bone effects (long-term) — Estrogen plays an important role in male bone homeostasis. Chronic estrogen receptor antagonism could theoretically affect bone; not formally quantified in long-term trials. Ensure adequate calcium, vitamin D, and weight-bearing exercise.
• Drug interactions — Limited characterized interactions. Possible interactions with other SERMs, aromatase inhibitors, estrogen replacement therapy. No major CYP-mediated interactions characterized at typical doses.
• WADA banned (S4) — Anti-estrogens including SERMs are banned at all times in tested athletes. Detection methods established. Any tested athlete using enclomiphene faces sanction.
• Contraindications — Primary hypogonadism (testicular failure — won't work), liver disease, prior VTE, pregnancy (men), women of reproductive age.

Bloodwork & Monitoring

Enclomiphene warrants standard endocrine monitoring on a regular cadence.

• Baseline (pre-treatment) — Total testosterone (morning), free testosterone, LH, FSH, estradiol (sensitive assay), SHBG, prolactin, TSH, lipid panel, CBC, CMP, PSA (men >40), HbA1c.
• 6-week recheck — Total and free testosterone, LH, FSH, estradiol, SHBG. Adjust dose based on labs and symptoms.
• 12-week recheck — Same panel plus lipid panel, CBC, CMP. Confirm steady-state response and tolerability.
• Every 3–6 months thereafter — Total T, free T, LH, FSH, estradiol; annually full panel including PSA in men >40.
• Sperm count (if fertility-relevant) — Baseline and at 6 and 12 weeks for men trying to conceive. Should be preserved or improved on enclomiphene (vs reduced on TRT).
• Bone density (DXA) — Consider baseline and biennial monitoring on long-term (12+ month) chronic use given the theoretical bone concern.
• Blood pressure and weight — Routine; not enclomiphene-specific.
• Subjective symptom tracking — Energy, libido, morning erections, mood, sleep quality. Standardized scales (ADAM, AMS, IIEF-5) useful for objective tracking.

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Regulatory Status

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Key References

1. Hill S, Arutchelvam V, Quinton R. Enclomiphene, an estrogen receptor antagonist for the treatment of testosterone deficiency in men. IDrugs. 2009;12(2):109-119. (Foundational enclomiphene clinical pharmacology paper.)
2. Wiehle RD, Fontenot GK, Wike J, Hsu K, Nydell J, Lipshultz L. Testosterone restoration using enclomiphene citrate in men with secondary hypogonadism: a pharmacodynamic and pharmacokinetic study. BJU Int. 2013;112(8):1188-1200. PMC4155868. (PK/PD study; 48 men; 6 weeks.)
3. Wiehle RD, Fontenot GK, Hsu K, Lipshultz L. Oral Enclomiphene Citrate Stimulates the Endogenous Production of Testosterone and Sperm Counts in Men with Low Testosterone: Comparison with Testosterone Gel. J Sex Med. 2014;11(5):1208-1218.
4. Kim ED, McCullough A, Kaminetsky J. Oral enclomiphene citrate raises testosterone and preserves sperm counts in obese hypogonadal men, unlike topical testosterone: restoration instead of replacement. BJU Int. 2016;117(4):677-685. PMID: 26496621. (Pivotal Phase 3 RCT data.)
5. Earl JA, Kim ED. Enclomiphene citrate: A treatment that maintains fertility in men with secondary hypogonadism. Expert Rev Endocrinol Metab. 2019;14(3):157-165. (Modern clinical review.)
6. Kaminetsky J, Werner M, Fontenot G, Wiehle RD. Oral enclomiphene citrate stimulates the endogenous production of testosterone and sperm counts in men with low testosterone: comparison with testosterone gel. J Sex Med. 2013;10(6):1628-1635.
7. Wiehle RD, Fontenot GK. Oral enclomiphene citrate lowers IGF-1 in men with secondary hypogonadism while raising testosterone: implications for cancer prevention. Cancer Res. 2013;73(8 Supplement):1326. (AACR abstract.)
8. Pastuszak AW, Wiehle RD, Hsu K, Lipshultz LI. Enclomiphene Citrate for the Treatment of Secondary Male Hypogonadism. Expert Opin Investig Drugs. 2016;25(10):1233-1240. PMC5009465. (Comprehensive clinical review.)
9. Helo S, Ellen J, Mechlin C, Feustel P, Grossman M, Ditkoff E, McCullough A. A randomized prospective double-blind comparison trial of clomiphene citrate and anastrozole in raising testosterone in hypogonadal infertile men. J Sex Med. 2015;12(8):1761-1769.
10. Bhasin S, Brito JP, Cunningham GR, Hayes FJ, Hodis HN, Matsumoto AM, Snyder PJ, Swerdloff RS, Wu FC, Yialamas MA. Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2018;103(5):1715-1744.
11. Repros Therapeutics. Press release: Repros Therapeutics Announces Cancellation of FDA Advisory Committee Meeting to Review Enclomiphene for the Treatment of Secondary Hypogonadism. October 29, 2015.
12. Drugs.com. Androxal Development History (FDA Approval: No; CRL December 2015). drugs.com/history/androxal.html.
13. European Medicines Agency. CHMP refusal of marketing authorization for EnCyzix (enclomifene). January 2018.
14. Adashi EY. Clomiphene citrate: mechanism(s) and site(s) of action—a hypothesis revisited. Fertil Steril. 1984;42(3):331-344.
15. Guay AT, Jacobson J, Perez JB, Hodge MB, Velasquez E. Clomiphene increases free testosterone levels in men with both secondary hypogonadism and erectile dysfunction: who does and does not benefit? Int J Impot Res. 2003;15(3):156-165.
16. FDA. Bulk Drug Substances Nominated for Use in Compounding — 503A and 503B Categories. FDA.gov. Updated 2025–2026.
17. WADA Prohibited List 2026. World Anti-Doping Agency. wada-ama.org.
18. ClinicalTrials.gov NCT01406964, NCT01403038 — Repros Phase 3 ZA-203 / ZA-204 trials.