The question almost nobody asks in time
A 38-year-old man comes in with fatigue, less strength at the gym, and lower sex drive. His labs show low testosterone. He's offered a gel or an injection. He starts, and within a few weeks he feels better.
Eight months later, he and his partner decide to try for a baby. And that's when the question nobody asked at the beginning shows up.
Because the testosterone he applied externally didn't just raise his blood level. It also switched off a signal.
That signal is the conversation between the brain and the testicles. The hypothalamus and the pituitary measure how much testosterone is circulating and, based on that, send two messengers: LH and FSH. LH tells the testicle to produce testosterone. FSH is part of producing sperm.
When testosterone arrives from outside, the brain reads a high level and draws the logical conclusion: there's enough already, no need to ask for more. LH drops. FSH drops. The testicle stops getting the order.
The result is that the blood level goes up while the factory shuts down. And the factory wasn't only making testosterone.
That's what this article is about: two paths that raise the same lab number through opposite mechanisms, and why that difference matters far more than it first appears.
What enclomiphene is
Enclomiphene is a selective estrogen receptor modulator. It's taken by mouth, as a pill.
The name sounds complicated, but the idea is simple. Part of a man's testosterone converts into estradiol, an estrogenic hormone. That estradiol travels to the brain and is one of the signals telling the hypothalamus "there's enough, turn the volume down."
Enclomiphene sits on that receptor in the brain and blocks the message. The hypothalamus stops receiving the brake signal. So LH rises, FSH rises — and the testicle gets more instruction to work.
Notice the direction. External testosterone enters the system at the end and shuts off the beginning. Enclomiphene enters at the beginning and switches on the end.
That's why the authors of one trial used a phrase that sums up the whole topic: restoration instead of replacement.
Why the isomer matters
There's a detail of chemistry that explains why this medication exists at all, rather than just the one that was already available.
Clomiphene has been used off-label for decades in men with low testosterone. But clomiphene isn't one molecule: it's a mixture of two mirror-image forms, enclomiphene and zuclomiphene.
They look alike on paper and behave very differently in the body. Enclomiphene is the one that blocks the receptor and raises gonadotropins. Zuclomiphene has a more estrogenic effect and stays in the body much longer.
A mouse study published in 2015 separated the two isomers and dosed them individually over a long period. The effects on Leydig cells, epididymis, seminal vesicles and kidneys appeared only with zuclomiphene. The isolated enclomiphene isomer raised testosterone with no effect on testicular histology.
It's an animal study, at high doses, and it should be read as exactly that. But it explains the reasoning: if the benefit comes from one isomer and a good share of the trouble from the other, it makes sense to try giving only one.
Replace or restore: the thermostat
The most useful image I know for this is a thermostat.
Picture a cold house. You have two ways to fix it.
The first is to bring in a space heater and switch it on in the living room. The house warms up. It works, and it works fast. But the thermostat on the wall senses that warmth, concludes the house is fine, and shuts off the central heating. As long as the space heater is running, everything looks normal. The day you turn it off, the house will be colder than before — because the central system hasn't run in months.
The second way is to adjust the thermostat. You're not adding new heat: you're changing what the system reads, so the house's own heating turns itself on.
External testosterone is the space heater. Enclomiphene is the thermostat adjustment.
Everything else follows from that. Both warm the house. Only one keeps the central system running. And in the body, that central system also makes sperm.
The trial numbers
This isn't theory. It was measured.
The first comparative study was published in 2013 in The Journal of Sexual Medicine. It was small and open-label: 12 men with secondary hypogonadism who had previously used topical testosterone. They were randomized to oral enclomiphene or testosterone gel and followed for 6 months.
At baseline, after stopping the prior gel, average total testosterone was 165 ± 66 pg/dL. At 6 months:
- Testosterone gel: 525 ± 256 pg/dL
- Enclomiphene: 545 ± 268 pg/dL
Essentially identical. If you only looked at the testosterone number on the lab report, you couldn't tell the two treatments apart.
The difference showed up everywhere else. Only the enclomiphene group showed increases in LH and FSH. And in sperm count the separation was clear: enclomiphene raised counts in 7 of 7 men at 3 months and 6 of 6 at 6 months, with concentrations between 75 and 334 million per mL. The gel failed to raise counts above 20 million per mL in any of the 5 men at 3 months.
Twelve people is very few. But the pattern repeated in larger studies.
In 2014, a phase II trial published in Fertility and Sterility confirmed the same thing: enclomiphene raised morning testosterone similarly to topical gel, while raising LH and FSH and conserving sperm counts.
And in 2015 came the two phase III trials, published together in BJU International. These were parallel randomized, double-blind, double-dummy studies in overweight men aged 18 to 60 with secondary hypogonadism confirmed on two separate measurements. They compared two doses of enclomiphene against 1.62% testosterone gel over 16 weeks.
The result was written plainly: total testosterone rose in all groups. LH and FSH rose on enclomiphene and fell on the gel. And enclomiphene maintained sperm concentration in the normal range across the treatment period, while the gel group showed a marked reduction in spermatogenesis.
| What the trials found | Enclomiphene | Testosterone gel |
|---|---|---|
| Total testosterone | Rises | Rises |
| LH and FSH | Rise | Fall |
| Sperm count | Maintained in normal range | Marked reduction |
| Route | Oral pill | Gel on the skin |
| FDA approval | Not approved | Approved |
That last row is worth sitting with, because it's uncomfortable and it's real. The treatment with the better fertility profile in these trials is the one without approval. We come back to that below.
What happens to sperm count on testosterone
Here you have to be precise, because the internet offers both opposite exaggerations.
One says testosterone makes you permanently sterile. The other says nothing happens and it sorts itself out right away. Neither is correct.
The best numbers come from an unexpected place: hormonal male contraception research. For years, androgens were studied precisely to suppress sperm production on purpose. That produced the cleanest data that exists on how long it takes to come back.
An integrated analysis published in The Lancet in 2006 pooled 1,549 healthy men from 30 studies, with 1,283.5 man-years of treatment and 705 man-years of post-treatment follow-up. Semen was measured every month until recovery.
The median time to return to 20 million per mL was 3.4 months. Probability of recovery was:
- 67% within 6 months
- 90% within 12 months
- 96% within 16 months
- 100% within 24 months
Read that with both hands. On one side it's reassuring: in that group, reversibility was complete within a predictable window. On the other side, a year is a year. If you're trying to conceive at 39, waiting twelve months for the system to switch back on is not a small detail.
And there's an important caveat. Those were healthy young men with normal baseline production and controlled treatments of limited duration. That is not the same as someone four years into testosterone therapy, or someone who used gym-level doses far above therapeutic range.
A 2025 review on testosterone use and abuse puts it without decoration: suppression of the axis lowers testosterone inside the testicle, impairs spermatogenesis, and can lead to testicular atrophy and azoospermia — and recovery after stopping is often prolonged and in some cases incomplete.
A Japanese clinical series published in 2020 shows the practical version. Of 4,375 men seen for infertility evaluation, 6 had received testosterone for late-onset hypogonadism. In 5 of the 6, gonadotropins were markedly suppressed, and semen findings ranged from severe oligospermia to azoospermia. Testosterone was stopped in all of them. Those needing prompt recovery received hCG or clomiphene and improved — except one who was treated for only a month.
That group's conclusion was direct: testosterone is contraindicated in men who want to preserve future fertility, and men who wish to preserve it should be considered for clomiphene or hCG.
Enclomiphene versus clomiphene
If clomiphene already exists, is inexpensive and is available, the obvious question is whether the single isomer is worth it.
A retrospective study from the University of Miami, published in 2023, compared the two in real-world practice: 46 men on enclomiphene and 32 on clomiphene, all treated at least three months. Median age was 42 and 41.
Both groups raised total testosterone significantly. But only enclomiphene significantly raised FSH and LH. And in total motile sperm count — the number a fertility specialist cares about most — only enclomiphene showed a significant increase.
Semen volume and concentration did not change significantly with either. Motility rose in both groups.
This is a retrospective, non-randomized, single-center study. It does not demonstrate superiority the way a head-to-head trial designed for that purpose would. But it points the same direction as the mouse work: separating the isomer appears to bring an advantage, especially in gonadotropins.
What the FDA decided — and what it didn't
This is the part that almost never appears on the pages selling enclomiphene, and it's the part you should be clearest about.
Enclomiphene was developed as a branded product called Androxal. The company took it to the FDA three times between 2009 and 2015, and all three times received a Complete Response Letter — the agency's way of saying it will not approve the product as submitted.
In the last one, December 2015, the FDA stated that in light of recent scientific developments the design of the phase III studies was no longer adequate to demonstrate clinical benefit in the target population, and recommended one or more additional phase III studies. It also raised points about study entry criteria, dose titration, and bioanalytical method validation.
Those additional studies were never completed. Development stopped.
There was a second chapter. In June 2022, the FDA's Pharmacy Compounding Advisory Committee voted against including enclomiphene citrate on the final 503A Bulks List. The substance currently sits in Category 1 — under evaluation — and is not on the approved list.
In practical terms this means three things:
- There is no FDA-approved enclomiphene product. What's available in the United States comes from compounding pharmacies.
- A compounded medication does not go through the pre-market efficacy and safety review that an approved product goes through.
- Dose consistency between one pharmacy and another is not guaranteed in the same way.
None of this means enclomiphene doesn't work. The trials exist and they're consistent. It means the regulatory backing testosterone has, enclomiphene does not — and that asymmetry is part of the decision.
Saying it the other way around would be easier to sell. It would also be false.
What this evidence does NOT say
The studies above are real and published. They also have limits worth naming clearly.
They did not measure pregnancies. Every trial measured hormones and sperm counts. None measured whether more babies were born. A better count is a good indicator, but it is not the same as a fertility outcome.
They were short. Sixteen weeks in the phase III trials, six months in the small ones. Low testosterone is often treated for years. These trials tell us nothing about year five.
Several were funded by the manufacturer. The phase II and isomer studies have authors from the company developing the product. That doesn't invalidate the data, but it's information you should have while reading it.
There is no long-term cardiovascular safety data for enclomiphene. Testosterone has the TRAVERSE trial, with thousands of participants and cardiovascular event follow-up. Enclomiphene has nothing equivalent. Absence of data is not the same as absence of risk.
The isomer study was in mice. At high doses, by oral gavage. It's useful for understanding mechanism, not for predicting what happens in a person.
The clomiphene comparison was retrospective. Looking back through charts is not the same as random assignment. Differences between the patients who received one or the other could explain part of the result.
The recovery numbers come from healthy men. The Lancet analysis studied young eugonadal volunteers in controlled protocols. Your situation may not resemble theirs.
Who is this NOT for?
Enclomiphene is not the answer for every man with low testosterone. There are situations where it simply doesn't apply.
If the problem is in the testicle, not the brain. Enclomiphene works by raising LH and FSH so the testicle works harder. If the testicle can't respond — what's called primary hypogonadism — shouting louder doesn't help. In that scenario the mechanism has nothing to act on.
If preserving fertility isn't a concern and you want the path with more regulatory backing. For a 62-year-old man not planning children, enclomiphene's main advantage carries less weight, and approved testosterone has decades of use and a large cardiovascular trial behind it.
If you expect an immediate effect. Enclomiphene works through your own hormonal axis. That takes weeks, and it depends on the system having the capacity to respond.
If there's a treatable cause nobody has looked for. Undiagnosed sleep apnea, a pituitary problem, certain medications, prior anabolic steroid use, excess prolactin. Raising a number without understanding why it fell is treating the alarm instead of the fire.
If you're not an adult man. Nothing described here applies to women or to minors.
And one more, which applies to both treatments: if your testosterone is low partly because your body weight is pushing it down, that's a separate issue worth addressing in parallel. Fat tissue converts testosterone into estradiol, and that extra estradiol presses the brain's brake. It's no accident that the phase III trials were run specifically in overweight men.
So when is each one used?
There's no general answer. There's an answer per person. But the questions that determine it are fairly stable:
Do you want children, now or later? This is the heaviest question. If the answer is yes or maybe, the conversation changes entirely, and the evidence points toward preserving the signal rather than replacing it.
Where is the failure, upstream or downstream? Labs distinguish this. Low testosterone with low or normal LH and FSH points to a signal problem. Low testosterone with high LH and FSH points to a testicle that isn't responding.
How much does regulatory backing matter to you? This is a legitimate consideration in both directions, and people weigh it differently. What matters is making the decision knowing where things stand.
Is something else explaining the low level? Weight, sleep, alcohol, sustained stress, medications, prior steroid use. Sometimes the shortest path is neither medication.
If you want the fuller picture of what total and free testosterone measure, and what the TRAVERSE trial actually found, we cover that separately in men's hormonal health.