Enclomiphene

Enclomiphene, also known by its former development name Androxal, is a non-steroidal selective estrogen receptor modulator (SERM). It is the trans-isomer of clomiphene citrate, a decades-old fertility drug that is itself a mixture of two isomers: the trans-isomer enclomiphene and the cis-isomer zuclomiphene ^[3][5]. Research established that most of the beneficial, testosterone-raising activity of clomiphene comes from the enclomiphene fraction, while the longer-lived, more estrogenic zuclomiphene contributes little to the intended effect and lingers in the body; isolating the trans-isomer was therefore pursued to create a cleaner agent ^[4].

Enclomiphene was developed by Repros Therapeutics specifically for secondary (hypogonadotropic) hypogonadism in men, the situation in which the testes can still make testosterone but are not being adequately stimulated by the pituitary. Its central appeal is that it raises testosterone by working through the body's own hormonal axis rather than replacing the hormone from outside, which allows it to preserve fertility. This was tested in a proof-of-principle phase II study against testosterone gel and in two large phase III trials in overweight men with low testosterone ^[1][2].

Enclomiphene is taken orally as a tablet. It is not an approved medicine in the United States, where its new drug application was not ultimately approved, so it is encountered off-label, through compounding pharmacies, or as a research chemical; clomiphene itself continues to be used off-label for the same purpose. Within men's health and longevity circles it is discussed as a fertility-sparing alternative to testosterone replacement therapy ^[3][4].

Enclomiphene is a selective estrogen receptor modulator that raises LH, FSH and testosterone, and the short-term trial record is unremarkable. In the pivotal phase III programme 53 of the treated men, about 21 percent, had an adverse event attributed to study drug and none was severe or serious, with no significant difference in rate between enclomiphene, testosterone gel and placebo; the two discontinuations in the 25 mg arm were for a raised hematocrit and haemoglobin and for a raised PSA ^[16]. The phase II trial reported treatment-emergent events in 9 to 20 percent of subjects across all arms, and in the enclomiphene arms these amounted to one mildly raised estradiol, one mild sinus headache and one moderate headache, with no serious events and no attributable change in chemistry, hematology or urinalysis ^[16]. One death from stroke occurred in an enclomiphene arm in a man with multiple pre-existing risk factors ^[16].

Against the mixture the isomer looks gentler, though only retrospectively. In 66 men prescribed clomiphene and later enclomiphene at the same centre, decreased libido, reduced energy and mood change were all significantly less frequent on enclomiphene, and the odds of any recorded adverse event were lower by a factor of about five (odds ratio 0.18, 95% CI 0.07 to 0.44) ^[15]. That is a chart review, not a trial, and the second drug always follows the first, so a period effect cannot be excluded.

What is genuinely known versus what is borrowed matters here, because most of what circulates about this compound is borrowed. Visual disturbance is the classic warning for this drug class and the development programme treated it as such: the 12-month bone study monitored safety with visual acuity examinations, slit lamp and fundoscopy, and excluded men with symptomatic cataract or an abnormal fundoscopy at entry. No published enclomiphene trial has reported a visual adverse event, and no published enclomiphene trial has reported a venous thromboembolic event either; the single pulmonary embolism in the pooled randomized evidence occurred in an anastrozole arm, in a man with a prior deep vein thrombosis, not in a SERM arm ^[16]. Any thrombosis warning carried here is therefore extrapolated from clomiphene and tamoxifen labelling; it has not been observed with enclomiphene itself. On bone, six weeks of treatment left bone markers unchanged ^[12], and the sponsor's 52-week randomized bone mineral density study reported no evidence of a negative effect; that result was announced by press release in October 2014 and no results have been posted to the trial registry and no journal publication has been located, so it cannot be read as published evidence. Long-term human safety data specific to enclomiphene do not exist: the longest follow-up in any pooled randomized trial is 30 weeks, and the meta-analysis that pooled them says in its own words that it 'remains underpowered with regard to safety endpoints' ^[16].

Enclomiphene is the trans-isomer of clomiphene and shares that class's documented cautions; the clomiphene labeling records no clinically significant drug-drug interactions, so its main documented contraindications are pregnancy and pre-existing liver disease rather than specific co-medications. As a selective estrogen receptor modulator it may theoretically oppose or overlap with other estrogen-receptor agents such as estrogens or other SERMs, but this is a class-level theoretical concern and not an established interaction. There is no documented CYP-based interaction profile for the compound. Enclomiphene itself remains investigational and lacks an approved label with a dedicated interactions section. This is research information, not medical advice.

Checking a whole stack? Run it through interactions + stacks.

Enclomiphene is the purified trans-isomer of clomiphene, a selective estrogen receptor modulator first introduced in the 1960s as the fertility drug clomiphene citrate, long marketed to induce ovulation in women. Clomiphene is a mixture of two geometric isomers with opposing pharmacology: the estrogenic zuclomiphene, which clears slowly and accumulates, and the antiestrogenic enclomiphene, which is more rapidly eliminated. Recognizing that the antiestrogenic isomer carried the desirable action for stimulating the male reproductive axis, the Texas-based company Repros Therapeutics developed the isolated trans-isomer under the name Androxal for the treatment of secondary hypogonadism in men.

During the 2010s Repros conducted a program of phase II and phase III trials comparing oral enclomiphene against topical testosterone gel, establishing that it raised testosterone while preserving gonadotropins and sperm production. The regulatory ending is specific and worth knowing precisely. Repros submitted a new drug application in February 2015; the FDA accepted it for review on 1 April 2015 and set a user-fee goal date of 30 November 2015.

An advisory committee meeting was scheduled for 3 November 2015 and then cancelled by the agency's Division of Bone, Reproductive and Urologic Products because of questions raised late in the review about bioanalytical method validation that could affect how certain pivotal study data were interpreted. On 1 December 2015 Repros announced it had received a Complete Response Letter, in which the FDA stated that on recent scientific developments the design of the phase III studies was no longer adequate to demonstrate clinical benefit and recommended an additional phase III study or studies; the agency also raised concerns about study entry criteria, titration and bioanalytical method validation.

Repros met FDA reviewers and senior leaders on 4 February 2016 to discuss resolution, never ran the additional study, and stopped filing with the Securities and Exchange Commission in February 2018. The application was never approved. Separately, on 8 June 2022 the FDA's Pharmacy Compounding Advisory Committee voted 8 to 4 against placing enclomiphene citrate on the section 503A bulk drug substances list, the FDA having proposed that it not be included; several members voting no cited a lack of clinical efficacy evidence. It is nonetheless sold widely today through men's health telehealth clinics and compounding pharmacies.

Enclomiphene has earned a strong following among clinicians and patients focused on male hormonal health because it addresses a real limitation of conventional testosterone replacement: the suppression of fertility. Rather than supplying testosterone from outside, it prompts the body to make more of its own by lifting the estrogen brake on the pituitary, an approach often described as restoration rather than replacement.

Randomized comparisons against testosterone gel are frequently cited in its favor, since enclomiphene raised testosterone into the normal range while maintaining or improving sperm concentration, whereas the gel reduced it. It is taken as a simple oral tablet, which many find more convenient than injections or daily gels. Honesty requires noting that it lacks formal regulatory approval for this use and that long-term outcome data remain limited, but its clear mechanistic rationale and consistent short-term results give it a well-deserved reputation as a fertility-sparing option.