The explosion of interest in glucagon-like peptide 1 receptor agonists (GLP-1 RAs), such as semaglutide and tirzepatide, has raised questions about what therapeutic effects this class of medication might have beyond their current indications for type 2 diabetes and obesity.
Recent clinical trials have recently identified benefits from GLP-1 agents for the heart, liver, and kidneys, but the current evidence base is murkier regarding how the drugs may affect male fertility.
Experts say the connection between GLP-1 RAs and improved male fertility makes sense biologically. For starters, overweight and obesity are strongly associated with male infertility in several overlapping ways. Obesity can disrupt hormones linked to fertility, increase the risk for defective sperm, adversely affect semen quality, and even make sexual intercourse more difficult due to obesity’s link to erectile dysfunction. As a result, GLP-1 RAs should at least in theory boost male fertility in men who take the drugs to lose weight.
But animal studies and a handful of small trials and observational data point to the potential for GLP-1 RAs to improve male fertility in other ways.
A recent narrative review on GLP-1 RAs and male reproductive health, published in the journal Medicina in December 2023, surveyed the potential of the drugs for male infertility and offered reason for optimism.
Hossein Sadeghi-Nejad, MD, director of urology at the NYU Grossman School of Medicine and a co-author of the article, said that one reason he and his colleagues conducted their analysis was the known association between weight loss and an increase in testosterone.
“Most of the animal studies that are out there show that this class of drugs does affect testosterone levels,” Sadeghi-Nejad said; they wanted to better understand what other evidence showed about GLP-1 agonists and other fertility factors.
Link Between Obesity and Fertility
The recent paper first reviews the well-established link between obesity and poorer fertility outcomes.
“Certainly, obesity poses a significant societal problem with substantial impacts on both overall health and economic aspects,” senior author Ranjith Ramasamy, MD, an associate professor of urology and director of the Reproductive Urology Fellowship program at the University of Miami’s Miller School of Medicine, told Medscape Medical News. “The escalating global obesity rates raise concerns, especially in the field of male infertility, where excessive body fat induces intrinsic hormonal changes leading to alterations, eventually, in semen parameters.”
The authors noted that obesity has been linked in the research to worse assisted reproductive technology (ART) outcomes and to subfecundity, taking more than 12 months to achieve pregnancy. They also referenced a systematic review that found men with obesity were more likely to have lower sperm counts and less viable sperm.
“From our standpoint, I think the key point was to raise awareness about the fact that obesity, because of the aromatization of testosterone to estradiol [from excess adipose tissue], will affect the hormonal axis and the availability of testosterone and, therefore, indirectly affects spermatogenesis,” Sadeghi-Nejad said.
Obesity is also linked to lower levels of inhibin B, which stimulates testosterone secretion in Sertoli cells, which, when combined with the proinflammatory state of obesity, “results in a less favorable environment for sperm production,” he said. Finally, the link between obesity and poorer sexual function further inhibits fertility potential, he added.
Until recently, the primary treatments for obesity in men experiencing fertility problems have been lifestyle modifications or surgical interventions. But the recent approval of GLP-1 RA drugs for obesity present an additional option depending on how these drugs affect other fertility parameters.
Direct or Indirect Effects?
Most of the available evidence on GLP-RAs and sperm parameters comes from preclinical research. One of the few clinical trials, published last year in the Journal of Clinical Medicine, investigated the effects of liraglutide in men with metabolic hypogonadism, a body mass index between (BMI) 30 and 40, and severe erectile dysfunction.
Among the 110 men enrolled in the study, only the 35 participants who said that they were not seeking fatherhood received liraglutide. After four months of treatment, these men had significantly improved semen concentration, motility, and morphology than did those wanting to conceive who received conventional fertility treatment. Erectile dysfunction was also more improved in the liraglutide group, according to the researchers.
Though this study demonstrated the potential for liraglutide to treat metabolic hypogonadism, the men in that group also had greater weight loss and BMI reduction than the other participants. The review cited several other studies — albeit small ones — in which weight loss was associated with improvements in sperm parameters, including one randomized controlled trial in which one group lost weight with liraglutide and the other with lifestyle modifications; both groups showed increases in the concentration and number of sperm.
One of the key questions requiring further research, then, is whether GLP-1 agents have direct effects on male fertility independent of a reduction in obesity. The randomized controlled trials comparing liraglutide and lifestyle modifications failed to find additional effects on semen in the men taking liraglutide; however, the study had only 56 participants, and results from liraglutide cannot be generalized to potential effects of semaglutide or tirzepatide. Sadeghi-Nejad said.
“Determining the relative contributions of weight loss versus direct drug actions on fertility outcomes remains challenging without robust data,” Ramaswamy said. “While acknowledged that diet and physical activity positively impact fertility, confirming the synergistic role of GLP-1 receptor agonists requires evidence from well-designed randomized clinical trials.”
Rodent studies suggest that GLP-1 RAs may independently affect testicular function because GLP-1 receptors exist in Sertoli and Leydig cells of the testes. In one study, for example, obese mice who received the GLP-1 agonist exenatide for 8 weeks had “improved sperm motility, DNA integrity, and decreased expression of pro-inflammatory cytokines,” the authors of the review reported. But the precise mechanisms aren’t well understood.
“We know that there are GLP-1 receptors in the reproductive tract, but the extent of the downstream effect of stimulating those receptors, I don’t think we know well,” said John P. Lindsey II, MD, MEng, an assistant professor of urology at University of California San Francisco Health.
Other hormonal effects of GLP-1 agonists, such as stimulating insulin production and better regulating blood glucose levels, are better understood, said Raevti Bole, MD, a urologist at Cleveland Clinic, in Ohio, but still other effects of the drugs may not yet be identified.
“I think the really big unknown is whether these types of drugs have effects that are not hormonal on male fertility and what those effects are, and how those affect sperm,” Bole said. “For example, we know that these drugs slow gastric emptying. Is it possible that slow gastric emptying affects some of the nutrients that you absorb, and that could affect fertility?” Similarly, she said, it’s not clear whether GLP-1 agonists would have any effects on the thyroid that could then affect fertility.
Effects on Offspring
Another open question about GLP-1 RAs and male fertility is their potential effects on the offspring, said Sriram Machineni, MBBS, an associate professor of endocrinology at the Albert Einstein College of Medicine in New York City. The clinical trials involving the drugs for treating type 2 diabetes and obesity required both men and women to use contraception. If sperm contributing to a pregnancy are exposed to a GLP-1 agent, “we don’t know what the consequences could be,” Machineni said. “Just increasing the fertility of the man is not enough. We need to make sure it’s safe long-term for the fetus.”
Bole also pointed out the need for understanding potential effects in the fetus.
“We know that there are epigenetic changes that can happen to sperm that are influenced by the lifestyle and the physical health and environment of the parent,” Bole said. “So how could these drugs potentially affect those epigenetic changes that then potentially are passed on to the offspring? We don’t know that.”
An ideal source for that data would be a cohort registry of people who are taking the medication and then cause a pregnancy. “They have a registry for pregnant women,” Machineni said, “but we need something similar for men.”
Sadeghi-Nejad said that he and his co-authors are working on developing a registry for men who take GLP-1 RAs that would enable long-term tracking of multiple andrologic outcomes, including fertility and sexual dysfunction. Such a registry could theoretically be useful in tracking pregnancy and offspring outcomes as well.
Too Soon for Prescribing
Additional options for treating fertility in men with obesity would be welcome. Current treatments include the selective estrogen receptor modulator (SERM) clomiphene citrate and the aromatase inhibitor anastrozole. But these have their drawbacks, Sadeghi-Nejad pointed out; in the overweight population in particular, they “are not necessarily ideal,” he said.
“Although both are viable treatments for enhancing hormonal balance and semen parameters, clomiphene citrate has rare but documented side effects, including thromboembolism, gastrointestinal distress and occasional weight gain in men,” Sadeghi-Nejad and his colleagues wrote. “Furthermore, despite clomiphene citrate’s association with significant increases in sperm concentration, it is not universally effective, with a meta-analysis indicating a significant increase in sperm concentration in approximately 60% of men.”
For men who have obesity and oligospermia but normal levels of testosterone and estradiol, “conventional pharmaceutical approaches like clomiphene may not be suitable,” the authors wrote.
Still, GLP-1 RAs may have a role to play for this population.
“I think it is within the wheelhouse of a reproductive urologist to consider those types of medications,” Lindsey said. For example, for a patient who has overweight or obesity, “does it make sense to think about doing clomiphene therapy, which we often do for someone who has low testosterone, in conjunction [with a GLP-1 agonist]? Maybe there’s a kind of an additive effect of having both on board.”
Ramasamy similarly noted that GLP-1 agonists cannot replace SERMs but may work “synergistically” with them.
“Despite the established popularity of GLP-1 receptor agonists, there may be some reluctance among urologists and fertility specialists to prescribe them, with some others advocating for their use to enhance semen parameters,” Ramasamy said. “However, robust scientific evidence is still lacking, necessitating caution and a wait for more substantial data.”
Even if GLP-1 RAs prove to have therapeutic benefit for fertility, considerations such as availability and cost may affect prescribing.
“We do currently have safe and effective drugs that we use for male fertility, and those are generally nowhere near as expensive,” Bole said. “When we start talking about another drug that we can add, we have to think about the efficacy and the potential side effect but also, is this affordable for patients?”
Eventually, once more evidence become available, all of the urologists who spoke with Medscape Medical News said that they expect discussion about the possible therapeutic utility of GLP-1 agonists to make its way into clinical guidelines.
“Obesity is such a huge impediment for fertility in the modern environment,” Machineni said. “We will have to clarify the use of these agents, so I think this will be a part of the guidelines some point, but I think we need more information.”
The research was funded by the National Institute of Diabetes and Digestive and Kidney Diseases and the American Cancer Society. The review authors and other quoted physicians reported no disclosures. Dr. Machineni has consulted for Novo Nordisk and Lilly and has conducted clinical trials with semaglutide and tirzepatide for those companies.
Tara Haelle is a health/science journalist based in Dallas. Follow her at @tarahaelle.
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