Enclomiphene

The problem it is meant to solve

Anyone who supplies testosterone from outside switches off their own production. The brain measures the level in the blood, sees plenty available and shuts down the control hormones: LH, luteinizing hormone, and FSH, follicle-stimulating hormone. Without these two, two things stop happening in the testes. They no longer make their own testosterone, and they no longer make sperm.

Hence the reports of infertility and shrinking testes under testosterone therapy. Once family planning is complete, that is an acceptable price; for those who want children, it is a problem. This is exactly the gap enclomiphene targets.

How it works and where it comes from

The brain does not actually measure the testosterone level directly. It mainly measures estrogen: in the body, testosterone is partly converted into estradiol, and this estradiol presses on the feedback switch. Enclomiphene blocks this switch. The brain believes there is too little and turns up LH and FSH. The testes receive more signal and produce more – their own testosterone and sperm along with it. So what is replaced is not the hormone but the signal.

The substance itself is not new. Clomiphene has been approved for decades, though for inducing ovulation in women, and consists of two mirror-image forms. Zuclomiphene stays in the body for a very long time and acts more like an estrogen; enclomiphene is the anti-estrogenic half with a short half-life. Enclomiphene is thus purified clomiphene without the part that gets in the way.

Important for putting this in context: the principle requires an intact axis between hypothalamus, pituitary and testes. Where the testes themselves can no longer do the job, a stronger signal does not help.

What the studies show

Two phase 3 trials, 256 men, 16 weeks

Randomized, double-blind and double-dummy, with testosterone gel and placebo as comparators. Sperm concentration rose slightly under enclomiphene, by 11 to 15 percent, and fell under the gel by 33 to 57 percent. Under enclomiphene, 2 to 5 percent of men fell below the critical threshold of 15 million per milliliter; under the gel, 24 to 49 percent. Side effects were reported in 21 percent, none severe, with no difference between groups.

Review of 10 randomized trials, 819 men

Pools clomiphene and enclomiphene together. Compared with placebo, total testosterone rose by 274 ng/dl, confidence interval about 192 to 356. Compared with testosterone gel, the difference was 5.4 ng/dl with a p-value of 0.83, so practically zero: both approaches raise the level equally well. For sperm, by contrast, the gap is huge, namely 70.4 million per milliliter in favor of the tablet. The quality of evidence by GRADE is moderate for testosterone, LH and FSH versus placebo, downgraded because of wide confidence intervals and high heterogeneity.

TRAVERSE: the benchmark it has to be measured against

For classic testosterone therapy there is a trial with 5,246 men aged 45 to 80, with symptoms and cardiovascular risk, followed for about 33 months on average. For the combined cardiovascular endpoint, the figures were 7.0 percent under testosterone versus 7.3 percent under placebo, hazard ratio 0.96. There are signals there too: arrhythmias requiring treatment 5.2 versus 3.3 percent, acute kidney injury 2.3 versus 1.5 percent. The difference in scale remains: 5,246 men over 33 months versus 256 men over 16 weeks.

Where the data stop

On December 1, 2015, the US regulatory agency sent the manufacturer a rejection letter. The reasoning: the design of the phase 3 trials was no longer suitable for demonstrating a clinical benefit. Added to this were three objections concerning inclusion criteria, dose titration and the validation of the measurement method. The objection seems unfair at first, because the trials showed exactly what they were meant to show. But what is on the label is a lab value: testosterone rises, LH rises, FSH rises, sperm are preserved. Nobody takes a tablet to change a number on a lab report. What would need to be measured is whether the man has more energy, more libido, less depressed mood, more muscle mass, fewer bone fractures.

That is exactly what remains open to this day. In the review, 2 of 10 studies assessed symptom questionnaires at all. One found no improvement in the general sexual function index compared with placebo; another found better scores on a questionnaire on age-related complaints and in sexual desire. The authors themselves call their conclusions on symptoms preliminary, and on safety they write that the analysis is underpowered despite everything. What remains is a clean mechanism, a shifted intermediate value and an open question about the outcome.

Status, approval and legal

Enclomiphene is not approved as a medicine anywhere, not even in the US. What is offered there runs through compounding, because the substance is a component of the approved clomiphene. In Germany, clomiphene is approved, but for inducing ovulation in women; there is no approval for men with low testosterone. Online, the trade works as with peptides: declared as a research chemical, not for human consumption. The US Department of Defense lists both substances on its prohibited list because they keep turning up as testosterone boosters in food supplements, which is illegal. And clomiphene is on the World Anti-Doping Agency’s prohibited list under hormone and metabolic modulators. For anyone who is tested anywhere, this is not a gray area but a hit.

Safety

In the phase 3 trials, side effects were reported in 21 percent of participants, none severe and with no difference between groups. That sounds good but says little: observation lasted 16 weeks in 256 men, and the review considers its own safety analysis underpowered. Reported are visual disturbances as a rare event, mood swings and headaches. This leads to a point regularly twisted in forums: anyone who chooses enclomiphene because it is supposedly safer than testosterone is choosing the substance with the weaker safety data – not because it is more dangerous, but because nobody has looked long enough. Sourcing via the gray market adds to this, with unverified origin and unverified content. Low testosterone needs to be investigated, because it has causes that should be found, and that investigation is done by a physician, not by a provider who sells the remedy along with it.

Sources

• Kim et al., BJU International 2016 — two parallel phase 3 trials on enclomiphene (ZA-304, ZA-305), 256 men, 16 weeks
• Hohl et al., Archives of Endocrinology and Metabolism 2025 — review of 10 randomized trials on clomiphene and enclomiphene, 819 men
• TRAVERSE (Lincoff et al., NEJM 2023) – cardiovascular safety of testosterone therapy, 5,246 men, about 33 months
• Rejection letter from the US regulatory agency of December 1, 2015
• World Anti-Doping Agency prohibited list, hormone and metabolic modulators
• US Department of Defense prohibited list for dietary supplements