Enclomiphene: the monograph

Enclomiphene is the estrogen-blocking half of a 59-year-old fertility drug, sold on its own as a testosterone booster for men. It is real pharmacology with real randomized trials behind it: in the phase III program, oral enclomiphene restored morning testosterone to the normal range while keeping sperm concentration normal, the exact thing testosterone gel failed to do in the same trials⁴. That much of the sales pitch is true.

Here is the part the sales pitch omits. Those trials were run to win FDA approval for a drug called Androxal, and the approval never came. FDA cancelled the advisory committee vote days before the November 2015 meeting over bioanalytical questions¹¹, then issued a Complete Response Letter stating that the phase III program's design was no longer adequate to demonstrate clinical benefit¹². Europe went further: in January 2018 the EMA refused the same molecule (as EnCyzix) because raising testosterone was never shown to improve symptoms, and because of venous thromboembolism risk¹⁴. The developer was sold for $0.67 a share¹⁵. No regulator anywhere has approved enclomiphene¹⁹.

What exists today is a compounded market operating under an interim FDA policy while the agency evaluates enclomiphene citrate for the compounding bulks list, where it sits in Category 1, under evaluation, nominated in 2022 and still undecided¹⁶¹⁷. This monograph documents the whole record: every major trial with numbers, the isomer pharmacology that makes enclomiphene genuinely interesting, the two regulatory endings, and what remains unknown.

Clomiphene citrate, approved by FDA in 1967 as Clomid for ovulatory dysfunction in women, is not one molecule. The label states it plainly: clomiphene is a mixture of two geometric isomers, cis (zuclomiphene) and trans (enclomiphene), containing between 30% and 50% of the cis isomer²¹²⁰. The two isomers are pharmacological opposites: zuclomiphene acts as an estrogen agonist, enclomiphene as an estrogen antagonist¹⁸⁴².

Enclomiphene is that trans isomer isolated: C26H28ClNO, molecular weight 406.0, CAS 15690-57-0³⁰. It carries old development codes (RMI 16,289, ICI 46476) and one famous one: Androxal, the name under which Repros Therapeutics took it through phase III trials for secondary hypogonadism³⁰¹². In Europe the same product was filed as EnCyzix¹³.

The idea behind isolating the isomer is straightforward: keep the half of clomiphene that blocks estrogen feedback and raises gonadotropins, drop the half that is itself estrogenic and lingers in the body for months. Whether that idea ever earned an approved product is a separate question, and the answer so far is no¹⁹.

Enclomiphene by itself is not an FDA-approved drug (the approved related drug is mixed-isomer clomiphene). Enclomiphene is sold as a compounded product under an interim FDA policy, without FDA review of its safety or effectiveness.

The precise status: a Drugs@FDA search for enclomiphene returns no approved product¹⁹. Enclomiphene citrate appears on FDA's 503A list of bulk drug substances in Category 1, Bulk Drug Substances Under Evaluation, in the category list updated May 14, 2026¹⁶. Category 1 is not an approval and not a safety finding; it is an interim policy under which FDA does not intend to take action against compounding with the substance while its eligibility for the permanent bulks list is evaluated¹⁶²².

That evaluation has been running since at least June 8, 2022, when FDA convened its Pharmacy Compounding Advisory Committee to discuss enclomiphene citrate, nominated for one use: to increase serum testosterone, LH and FSH to normal levels in the treatment of secondary hypogonadism¹⁷. Four years later the substance is still listed as under evaluation¹⁶. FDA states that compounded drugs are not FDA-approved and do not undergo premarket review for safety, effectiveness or quality²². The approved route to this pharmacology remains the mixed-isomer parent: clomiphene citrate, NDA 016131, approved in 1967, with generic versions actively marketed, prescribed off-label to men at physician discretion²⁰.

Enclomiphene is not an obscure research chemical. It was a lead drug candidate with a full FDA-track development program, and the record of that program is public.

Repros Therapeutics developed enclomiphene as Androxal for secondary hypogonadism, the low-testosterone pattern of overweight men whose pituitary signaling, not testes, is the problem¹². The program produced randomized phase II trials¹²³ and two parallel double-blind, double-dummy phase III trials against AndroGel (ZA-304 and ZA-305)⁴⁵⁶. On February 2, 2015 the company submitted NDA 207959; FDA accepted it on April 1, 2015, set a PDUFA goal date of November 30, 2015, and scheduled its Bone, Reproductive and Urologic Drugs Advisory Committee to vote on November 3, 2015¹⁰¹². The Federal Register notice for that meeting still describes the product: enclomiphene citrate 12.5 mg and 25 mg capsules for secondary hypogonadism in fertile men younger than 60 with a BMI over 25¹⁰.

The meeting never happened. On October 29, 2015, FDA cancelled it, citing questions that arose late in the review regarding the bioanalytical method validation that could affect interpretability of certain pivotal study data¹¹. On December 1, 2015, Repros received a Complete Response Letter. In its own SEC filing, the company reported FDA's grounds: based on recent scientific developments, the design of the enclomiphene phase 3 studies is no longer adequate to demonstrate clinical benefit, with concerns about study entry criteria, titration and bioanalytical method validation; FDA recommended an additional phase III study or studies¹². Two further phase III trials the company had registered were completed but never posted results and were never published⁸⁹.

Europe delivered the clearest verdict. On January 25, 2018, the EMA's CHMP refused marketing authorisation for EnCyzix (enclomifene), reviewing 4 main studies in 588 patients. The refusal reasons are two sentences long and worth reading verbatim in the source: the studies showed testosterone rose but did not look at whether symptoms such as bone strength, weight gain, impotence and libido would improve, and there is a risk of venous thromboembolism¹⁴¹³. Six weeks before that opinion, Repros had announced its acquisition by Allergan for $0.67 per share¹⁵. The program ended there. The molecule did not: it moved to compounding pharmacies, where the same trials now sell it without the endings¹⁶.

The entire case for enclomiphene rests on a pharmacokinetic asymmetry inside clomiphene, and it is one of the best-documented facts on this page.

Enclomiphene has a serum half-life of about 7 hours¹. Zuclomiphene's half-life is measured in days, not hours¹⁸: the Clomid label reports that detectable zuclomiphene persisted for longer than a month in single-dose volunteers and that label from a radiolabeled dose was still appearing in feces 6 weeks after administration²¹. After a single 50 mg clomiphene dose, measured zuclomiphene exposure (AUC 1289 ng/mL x h) was about 20-fold enclomiphene's (65 ng/mL x h)²⁵. Across repeated monthly cycles, zuclomiphene accumulates progressively while enclomiphene is undetectable by day 3 of each cycle²⁶. In men taking clomiphene 25 mg daily long term, the median serum ratio of zuclomiphene to enclomiphene was 20:1²⁴. In an antidoping study, zuclomiphene remained detectable in urine 121 to more than 261 days after a single month of clomiphene²⁷.

So a man on chronic clomiphene is, at steady state, circulating mostly the estrogen-agonist isomer that contributes little to the therapeutic goal²⁴⁴². In male mice dosed with the separated isomers, zuclomiphene produced profound adverse effects on Leydig cells, epididymis and seminal vesicles, while isolated enclomiphene raised testosterone without testicular histology changes; the study's authors (Repros scientists, a conflict worth noting) argued this justified a mono-isomer product²⁸. Human head-to-head data point the same direction more gently: in 66 men switched from clomiphene to enclomiphene at one center, estradiol fell on enclomiphene while it had risen on clomiphene (-5.92 vs +17.50 pg/mL, p = 0.001), with fewer reported side effects (odds ratio 0.18), though the testosterone difference was not significant³⁹.

This is the honest version of the isomer argument: real, measured, human, and still not the same thing as proof of better clinical outcomes, which no trial has demonstrated³⁷.

In men, estrogen (made from testosterone by aromatase, abundantly in fat tissue) is the main brake on the hypothalamic-pituitary-gonadal axis. When estrogen occupies receptors in the hypothalamus and pituitary, the pituitary releases less LH and FSH, the testes get less stimulation, and testosterone falls. Overweight men with secondary hypogonadism are the textbook case: enough aromatase to keep the brake on, testes that still work if asked¹³⁴¹.

Enclomiphene is an estrogen receptor antagonist at that brake. Blocking hypothalamic and pituitary estrogen receptors releases gonadotropin secretion: LH and FSH rise, and the testes respond with more testosterone and continued spermatogenesis⁴¹¹. This is the opposite architecture from testosterone replacement, which raises serum testosterone from outside and thereby pushes LH and FSH down, shutting off the testes' own production and sperm output⁴²³.

The EMA's plain-language description of the same mechanism, written for the refused EnCyzix application: overweight men with hypogonadotropic hypogonadism produce too much oestrogen, which stops the release of gonadotrophins; EnCyzix blocks the excess oestrogen enabling release of gonadotrophins and normal testicular function¹⁴. The mechanism was never the dispute. What the mechanism delivers for how a man feels is where the record runs out¹⁴.

Enclomiphene's human evidence is unusually good for a compounded product, and unusually consistent about what it measured: hormones, not symptoms.

The phase II PK/PD trial (48 men randomized to enclomiphene 6.25, 12.5 or 25 mg daily vs AndroGel) found all three doses restored morning testosterone to the normal range; at week 6, testosterone on 25 mg was 604 +/- 160 ng/dL vs 500 +/- 278 ng/dL on the gel (no significant difference), with LH and FSH rising on enclomiphene while the gel suppressed them, and the effect persisting at least a week after stopping¹. The ZA-203 randomized trial confirmed the pattern and conserved sperm counts²⁷. In the small crossback trial of men coming off topical testosterone, enclomiphene elevated sperm counts in 7 of 7 men at 3 months (concentrations 75-334 million/mL) while the gel kept all 5 of its men below 20 million/mL³.

The phase III pair (ZA-304 and ZA-305, randomized, double-blind, double-dummy, 16 weeks, overweight men with testosterone at or below 300 ng/dL) delivered the program's signature finding: enclomiphene normalized total testosterone and maintained sperm concentration in the normal range, while testosterone gel raised testosterone but produced a marked reduction in spermatogenesis⁴⁵⁶. Meta-analysis of the randomized SERM evidence puts the placebo-adjusted testosterone gain at +273.76 ng/dL (95% CI 191.87-355.66) with no difference vs testosterone gel on the testosterone endpoint³⁷; in obese men specifically, pooled enclomiphene trials show +7.50 nmol/L with almost no heterogeneity³⁸.

Read the fine print with us, though. These trials measured testosterone, LH, FSH and semen parameters. None of them was designed to show that men felt better, got stronger, lost fat, or protected their bones, and the EMA refusal turned on exactly that absence¹⁴. Two later phase III trials completed and simply never reported⁸⁹. A 2026 British Society of Sexual Medicine position statement recognises enclomiphene as promising for secondary hypogonadism, particularly where fertility matters, while advising that use stay with experienced clinicians in specialist or research settings given the absent long-term data and unlicensed status; several of its authors are affiliated with men's telehealth providers⁴⁵.

The sperm-preservation claim is the best-supported part of the enclomiphene story, with one honest asterisk.

What is demonstrated: in randomized comparisons, men on enclomiphene kept sperm concentrations in the normal range while men on testosterone gel showed marked reductions⁴, and in the small crossback trial every enclomiphene-treated man measured had elevated counts at 3 and 6 months³. Mechanistically this is expected: FSH, the hormone spermatogenesis depends on, rises on enclomiphene and falls on exogenous testosterone¹²³. The AndroGel label itself warns that exogenous androgens may lead to azoospermia²³, and the Endocrine Society guideline recommends against testosterone therapy in men planning fertility in the near term⁴³.

The asterisk: preserved sperm concentration is a surrogate. No enclomiphene trial has reported pregnancies or live births as an endpoint, so preserves fertility in the literal sense (children conceived) has not been demonstrated, only preserved semen parameters⁴⁴⁶. In the retrospective infertility comparison, total motile sperm count rose significantly on enclomiphene (and not on clomiphene), but semen volume and concentration changed on neither⁴⁰. If conception is the goal, this is a conversation for a reproductive urologist, not a storefront.

The comparison that matters commercially, because one of these is a cheap approved generic and the other is a premium compounded product, has surprisingly modest direct evidence.

No randomized head-to-head trial of enclomiphene vs clomiphene has been published. What exists: a within-patient longitudinal series (66 men who took clomiphene, then enclomiphene) found no significant difference in testosterone rise (median +166 vs +98 ng/dL, p = 0.20), a genuine divergence in estradiol (falling on enclomiphene, rising on clomiphene, p = 0.001), and fewer reported adverse effects on enclomiphene (OR 0.18, 95% CI 0.07-0.44)³⁹. A parallel-group retrospective study in infertility patients found both drugs raised testosterone, while only enclomiphene significantly raised LH, FSH and total motile sperm count⁴⁰. A 2025 meta-analysis of the randomized trials of both drugs pools them as one SERM class and finds the class beats placebo by about 274 ng/dL of testosterone³⁷.

Against that, clomiphene brings what enclomiphene cannot: decades of documented use in men, including a 400-patient series with up to 7 years of therapy (88% eugonadal beyond 3 years, 8% side effects, no significant adverse events)³¹, 3-year cohorts with bone density improvement³², and a 1642-patient meta-analysis³⁶, all off-label but all on an FDA-approved, pharmacy-dispensed, quality-controlled tablet²⁰. The zuclomiphene burden is clomiphene's real, measurable cost²⁴. Whether that cost is worth a compounded product's loss of FDA-reviewed manufacture is the actual decision, and it is closer than either side's marketing admits.

Education, not prescription: enclomiphene has no FDA-approved label and therefore no approved dose. What follows is what the clinical program actually used.

The Androxal program studied 6.25 mg, 12.5 mg and 25 mg once daily by mouth¹. All three doses raised testosterone into the normal range in the phase II PK/PD trial, with dose-related exposure¹. The NDA filed with FDA covered 12.5 mg and 25 mg capsules¹⁰, and the phase III trials used those strengths against testosterone gel for 16 weeks⁴. Pooled obese-men trials span enclomiphene 12.5-25 mg daily for 1.5-4 months³⁸.

Two grounding facts for anyone reading compounded-product dosing pages. First, in the trials the lowest dose (6.25 mg) already normalized testosterone in most men; more was not clearly better hormonally¹. Second, the comparison drug at typical male off-label doses (clomiphene 25 mg daily or 25-50 mg every other day²⁴³³) delivers a similar testosterone response in every comparative dataset³⁹³⁷. Dosing decisions belong with a licensed prescriber who can see your labs.

Unknown at the standard that word usually implies. Enclomiphene has meaningful short-term trial safety data and zero FDA-reviewed safety data, because no application ever passed review¹²¹⁹.

What the trials showed: across the randomized program, enclomiphene did not significantly change TSH, ACTH, cortisol, lipids or bone markers over 6 weeks¹; materials presented at FDA's 2022 compounding advisory meeting summarize 953 men exposed to enclomiphene across 11 blinded trials vs 290 on placebo and 130 on testosterone gel, with hemoglobin and hematocrit rising more on testosterone gel and clinically insignificant PSA changes on enclomiphene¹⁸. Meta-analyses report no unexpected safety findings in pooled short trials³⁸³⁷.

What regulators flagged anyway: the EMA refusal names a risk of venous thromboembolism with the medicine¹⁴, consistent with the drug class; the related SERM clomiphene carries label warnings for visual symptoms (blurring, spots, flashes) that require stopping²¹, and long-term male safety data do not exist for enclomiphene specifically, because the longest controlled exposure was measured in months⁴³⁷. IGF-1 fell in the phase II trial, an unexplained signal the published paper itself highlights¹. Mood and vision effects are documented for mixed-isomer clomiphene in men at low rates (8% overall side effects in the 400-man series)³¹; whether pure enclomiphene meaningfully improves on that is suggested by one retrospective series, not proven³⁹.

No approved label exists for enclomiphene, so no authoritative contraindication list exists either; that absence is itself a fact to weigh¹⁹. The following is assembled from the related approved drug's label and the trials' own exclusions, and a prescriber should make every one of these calls.

• History of blood clots. The EMA refused enclomiphene partly over venous thromboembolism risk¹⁴; testosterone products carry a VTE warning as well²³.
• Liver disease. The clomiphene label contraindicates use in liver disease or history of liver dysfunction²¹.
• Pituitary or intracranial lesions. The clomiphene label contraindicates use with organic intracranial lesions such as pituitary tumor; low testosterone caused by a pituitary tumor needs diagnosis, not masking²¹⁴³.
• Uncontrolled thyroid or adrenal dysfunction. Contraindicated on the clomiphene label²¹.
• Women and anyone pregnant. The male program has no female data; clomiphene itself is contraindicated in pregnancy²¹.
• Primary (testicular) hypogonadism. Mechanistically, raising LH and FSH cannot stimulate testes that cannot respond; the trials enrolled secondary hypogonadism only (LH below 9.4 IU/L)⁴⁴¹.

The formal interaction record is thin, which is different from safe.

What is documented: CYP2D6 is primarily responsible for metabolizing enclomiphene; in human liver microsomes, the CYP2D6 inhibitor quinidine completely blocked its metabolism, and microsomes from a CYP2D6 poor metabolizer showed no metabolism at all²⁹. That makes strong CYP2D6 inhibitors (some antidepressants among them) and CYP2D6 poor-metabolizer status plausible sources of higher exposure, a laboratory finding no clinical outcome study has followed up²⁹. Repros registered a dedicated Androxal drug-interaction study with CYP probes (NCT01991327); the registry marks it completed with no results posted, so its answers are not public⁴⁹.

Practical reading: bring an exact medication list to whoever prescribes any SERM, flag antidepressants and antipsychotics specifically for the CYP2D6 question²⁹, and treat any vendor that does not ask about your medications as having answered a different question, namely how much diligence to expect from them²².

Enclomiphene as a standalone product was developed for men only; the EMA application, the FDA NDA and every trial on this page enrolled men¹⁰¹³. There is no female dosing, efficacy or safety dataset for the isolated isomer.

The mixed-isomer parent, clomiphene, is the one with an approved female indication: ovulatory dysfunction in women desiring pregnancy, under physician supervision with specific warnings (ovarian hyperstimulation syndrome, visual symptoms, multiple gestation)²¹. Clomiphene is contraindicated during pregnancy, and the label notes the long zuclomiphene residence time means active drug may remain in the body during early pregnancy in women who conceive during a treatment cycle²¹. A zebrafish developmental study associating clomiphene and enclomiphene exposure with cranial suture changes exists; it is a fish model and proves nothing about human pregnancy, which is exactly why the pregnancy contraindication and washout pharmacology deserve respect²¹.

No enclomiphene label directs monitoring, so the honest sources are what the trials measured and what labels of the alternatives require.

The trials tracked morning total testosterone, LH, FSH, estradiol, SHBG, semen parameters, PSA, hematocrit and lipids¹⁴¹⁸. Estradiol genuinely moves on SERMs: it rose on clomiphene in men (17 to 34 pg/mL in the long-term series²⁴; significantly elevated in the 400-man series³¹) and fell modestly on enclomiphene in the switch study³⁹, so estradiol belongs on any monitoring panel. Hematocrit rose more on testosterone gel than enclomiphene in the pooled program data¹⁸; the AndroGel label's schedule (hematocrit at baseline, 3 to 6 months, then annually) is the class standard worth borrowing, and the Endocrine Society guideline lists elevated hematocrit among reasons not to start testosterone at all²³⁴³. PSA rose with endogenous testosterone raised by clomiphene in an early series, so age-appropriate prostate monitoring applies to this route too⁴⁸¹⁸.

Symptoms deserve equal billing: the EMA refusal reminds anyone reading that a rising testosterone number was never shown to translate into feeling better¹⁴. If a monitored trial of therapy does not improve the symptoms it was started for, the number alone is a poor reason to continue.

Whatever the pharmacy put in it, verified to whatever standard that pharmacy holds. That is not rhetoric; it is the regulatory design. Compounded drugs are not FDA-approved, and FDA does not review their safety, effectiveness or quality before marketing²². The 503A pathway requires a licensed pharmacy and a prescription, and enclomiphene citrate is compoundable today only because it sits in an interim under-evaluation category¹⁶.

The market's edges show what that means in practice. A 2026 published case series describes a compounded sublingual product combining enclomiphene 25 mg with boron, spermidine, vitamin C and a proprietary mineral-oxide delivery system, sold through a longevity practice; its own authors concede that causality and even basic axis effects cannot be determined from the data⁴⁷. Multi-ingredient blends, sublingual routes and dose claims outside the studied 6.25-25 mg range¹ all lack any trial basis. If you and your prescriber choose compounded enclomiphene, a state-licensed pharmacy, a US address, a prescription requirement and a certificate of analysis on request are the minimum diligence floor²².

Oral enclomiphene is absorbed with a tmax around 3 hours and cleared with a serum half-life of about 7 hours²⁵¹, metabolized primarily by CYP2D6 in vitro²⁹. Five half-lives, the conventional washout yardstick, is roughly a day and a half. Zuclomiphene, the isomer enclomiphene products leave out, has a half-life measured in days¹⁸, remains serum-detectable beyond a month after single doses²¹, accumulates across monthly cycles²⁶, reaches a 20:1 serum ratio over enclomiphene on chronic mixed-isomer therapy²⁴, and stays urine-detectable for 4 months or more²⁷. The label attributes the persistence pattern to possible stereo-specific enterohepatic recycling or sequestering²¹. This asymmetry is the entire pharmacokinetic argument for the isolated isomer, and it is well documented even though the clinical superiority it implies has never been shown in a randomized trial³⁷.

What we do not know yet, as of this writing. No enclomiphene trial has demonstrated that any symptom of low testosterone improves: not energy, mood, libido, erectile function, body composition, strength or bone density. The EMA refusal turned on precisely this gap, and no study since has closed it. Long-term safety is unmeasured: the longest controlled exposures ran months, the two later phase III trials completed without ever publishing results, and cardiovascular, thromboembolic and mood outcomes in men treated for years simply have no dataset. Fertility preservation means preserved semen parameters in trials up to 16 weeks; no pregnancy or live-birth outcomes have been reported. The comparison that would settle the premium over generic clomiphene, a randomized head-to-head trial, has never been run; the best available comparisons are retrospective. Effects on IGF-1 (which fell in the phase II trial) are unexplained. Compounded product quality is outside FDA review entirely, so batch-to-batch consistency data do not exist in the public record. And the regulatory evaluation that currently permits compounding is exactly that, an evaluation: FDA has been weighing enclomiphene citrate for the 503A bulks list since 2022 and could conclude it in either direction.