Letrozole, Known as a Female Ovulation Inducer, Shows Potential for Male Infertility in RCT
296 Men with Spermatogenic Dysfunction Treated for 3 Months… Sperm Concentration Stage Improvement of 14.3% vs. 5.4%
Sperm Identified in Ejaculate of Some Azoospermic Men… Though It Does Not Mean ‘Sperm Counts Increased Dramatically’
To couples undergoing fertility treatment, ‘letrozole’ is not an unfamiliar medication. It is commonly prescribed to induce ovulation in women. But what happens if this medication is administered to men who have virtually no sperm or no visible sperm at all?
Intriguing results have emerged.
A research team across 10 male infertility centers, led by Shanghai General Hospital at Shanghai Jiao Tong University School of Medicine, randomized 296 men with spermatogenic dysfunction into two groups—one receiving letrozole and the other not. After 3 months, the proportion of men whose sperm concentration improved by one stage or more was significantly higher in the letrozole group.
This study is noteworthy in that it evaluated a hormonal treatment long used empirically in male infertility through a large-scale randomized controlled trial (RCT).
Why Give a Drug Prescribed to Women to Men?
Letrozole is an ‘aromatase inhibitor.’ In the male body, aromatase is an enzyme involved in converting testosterone into estradiol, a female sex hormone.
Letrozole inhibits this enzyme. Put simply, it partially blocks the pathway through which testosterone converts to estradiol.
Consequently, estradiol levels decrease, while follicle-stimulating hormone (FSH) and luteinizing hormone (LH) secreted by the pituitary gland, as well as testosterone, can increase. FSH is the primary hormone that stimulates spermatogenesis in the testes.
Based on this mechanism, aromatase inhibitors like letrozole have already been prescribed off-label for select male infertility patients. The problem was the evidence: high-quality clinical trial data demonstrating whether it genuinely improves sperm parameters was lacking.
296 Men with Spermatogenic Dysfunction Were Randomized
From July 2023 to March 2024, the researchers recruited 296 men with spermatogenic dysfunction across 10 male infertility centers in China. The mean age was 30.2 years.
The participants were not mild male subfertility cases. They included patients with non-obstructive azoospermia (NOA), cryptozoospermia, and severe oligozoospermia. In particular, 218 men—73.6% of the cohort—had non-obstructive azoospermia.
Non-obstructive azoospermia differs from obstructive azoospermia, where sperm is produced but cannot exit due to blocked pathways. In NOA, no sperm is identified in the ejaculate because of impaired sperm production within the testes themselves.
The researchers administered letrozole 2.5 mg once daily for 3 months alongside vitamins C and E to 147 men. The control group of 149 men received vitamins C and E alone.
14.3% vs. 5.4%… A Difference Emerged
Comparing the results after 3 months revealed a clear difference.
The proportion of men who improved by one stage or more in the World Health Organization (WHO) sperm concentration classification was 21 out of 147 (14.3%) in the letrozole group, compared to 8 out of 149 (5.4%) in the control group.
The absolute difference was 9.2 percentage points, which was statistically significant.
According to the researchers’ calculations, treating approximately 11 men resulted in 1 additional patient achieving a sperm concentration stage improvement compared to the control group.
Furthermore, when evaluating the overall distribution of sperm concentration categories, the letrozole group had a significantly higher likelihood of reaching a better clinical stage than the control group.
However, an important caveat must be noted.
These findings should not be interpreted as meaning that “letrozole dramatically increases sperm counts in most men.” When comparing individual raw semen parameters between the two groups, some metrics showed no statistically significant difference. The essence of the finding is not that average sperm counts exploded across the board, but that a subset of men crossed into clinically meaningful ‘sperm concentration stages.’
Moving from Azoospermia to the ‘Visible Sperm’ Stage
The subgroup with non-obstructive azoospermia drew particular attention.
Among 105 men with non-obstructive azoospermia in the letrozole group, sperm was identified in the ejaculate of 11 men (10.5%). In the control group, it was found in 6 of 113 men (5.3%).
While this difference was not the primary endpoint of the study and involved a small sample size—meaning it cannot be concluded that letrozole “cures” azoospermia—the clinical significance remains notable. If sperm appears in the ejaculate of a man who previously had none, it introduces the possibility of avoiding invasive surgical sperm retrieval procedures, such as testicular sperm extraction (TESE).
This is precisely why the researchers placed greater emphasis on whether a patient’s ‘clinical stage changed’ rather than simple mean sperm counts.
Could Men Limited to ICSI Now Consider IVF or IUI?
Another compelling aspect of this study involves expanded ‘treatment options.’
When a man’s sperm count and motile sperm count are exceedingly low, natural conception or intrauterine insemination (IUI) is difficult to attempt, and in vitro fertilization (IVF) typically requires intracytoplasmic sperm injection (ICSI), where a single sperm is injected directly into each egg.
Conversely, if sperm concentration and total motile sperm count (TMSC) rise above certain thresholds, available clinical pathways can shift.
In fact, the rate of improvement in clinical grades for total motile sperm count was also higher in the letrozole group at 4.1%, compared to 0.7% in the control group. The researchers noted that such changes could open the possibility for some patients to consider less invasive options like standard IVF or IUI rather than ICSI.
Hormones Shifted as Expected
The physiological mechanism of the drug in the body was also confirmed.
Men taking letrozole showed a decrease in estradiol and an increase in gonadotropins (FSH and LH) as well as testosterone.
In other words, biological confirmation was established that letrozole shifted the hormonal milieu in a direction favorable to spermatogenesis.
Notably, the researchers reported that therapeutic efficacy was observed consistently not only in men with a low testosterone-to-estradiol ratio—who were historically considered the primary candidates for letrozole—but also in men with a normal baseline ratio. This finding could influence how candidate selection is approached in future clinical practice.
This Does Not Mean It Is an Established Treatment for Male Infertility
Clear limitations remain.
The trial lasted for 3 months. Beyond observing whether sperm parameters improved, the study did not evaluate whether actual pregnancy or live birth rates increased. Because the majority of participants were Han Chinese men, further verification is needed to determine whether identical outcomes appear in other populations and regions.
Side effects were also observed. Decreased libido was reported more frequently in the letrozole group at 12.2%, compared to 5.4% in the control group.
Therefore, it is inappropriate for men with low sperm counts to arbitrarily self-medicate with letrozole based on these results. Male infertility stems from diverse etiologies, including varicoceles, genetic anomalies, endocrine disorders, testicular dysfunction, and medications.
Nevertheless, this study provides a comparatively robust initial answer to an essential question:
“Can the testicular microenvironment for sperm production be pharmacologically modified in men who produce zero or minimal sperm?”
At least for a subset of men, the potential exists.
Whether a medication originally used to aid female ovulation can serve as a treatment that moves men from the “invisible sperm stage” to the “visible sperm stage”—and whether that change ultimately leads to pregnancy and childbirth—is the next question waiting to be answered.
※ Study Source: Based on a multicenter randomized clinical trial published in June 2026 in the international journal JAMA Network Open, conducted by researchers from Shanghai General Hospital at Shanghai Jiao Tong University School of Medicine and affiliated centers. Title: “Letrozole Treatment and Sperm Concentration in Men With Spermatogenic Dysfunction.” The study compared the efficacy of letrozole in 296 men with spermatogenic dysfunction. DOI: 10.1001/jamanetworkopen.2026.18422.
※ This article provides informational coverage of recent medical research and does not replace individualized diagnosis or treatment. Hormonal therapies for male infertility, including letrozole, must be evaluated and decided in consultation with a fertility specialist or urologist based on semen analysis, endocrine panels, and comprehensive causal evaluations.
※ The images used in this article were generated using artificial intelligence (ChatGPT, OpenAI) as illustrative reference materials and do not depict real individuals.
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