More than 9 percent of American males between the ages of 15 and 44 suffer from some form of infertility, according to data from the U.S. National Center for Health Statistics, part of the Centers for Disease Control and Prevention. That figure climbs to roughly 12 percent of all males between the ages of 25 and 44.

Male infertility can arise from a number of causes, some of which are more readily treatable than others. Help for some infertile men may soon come from an unlikely source — the oral erectile dysfunction drugs known as PDE5 inhibitors.

May Improve Sperm Quality

According to a recent review of the scientific literature, the active ingredients in these oral ED medications appear to improve the quality of Sperm in men considered infertile because of low sperm counts. Specifically, the drugs seem to increase the motility — spontaneous movement — of the sperm while also normalizing its shape.

Both the motility and shape of sperm are considered key factors in its ability to fertilize a female egg and create a pregnancy. Sperm that fails to move as rapidly as it should and is abnormally shaped is unlikely to achieve that goal.

The findings of the PDE5 inhibitors’ effects on Sperm Motility and morphology — the shape and structure of living organisms — come from a recently published Chinese review of 11 studies covering more than 1,300 men, many of whom were considered infertile. That review was previewed online in March 2017 in advance of its anticipated publication in a print issue of “Urology.”

Affects Only Infertile Men

Interestingly, the ED drugs’ salutary effects on sperm were observed only in infertile men, while no changes were seen in the sperm of those who were already fertile. The review of recent research was conducted by a team of scientists in the Department of Urology at China’s Sichuan University, located in Chengdu, the capital of Sichuan province.

In their review of recent relevant research, the Sichuan University urologists also noted that the most significant improvements in both sperm motility and morphology were observed in men who took PDE5 inhibitors regularly. This means that infertile men who pop ED drugs only once in a great while are unlikely to derive meaningful benefits as those who take them on a regular basis.

No Increase in Sperm Volume

While regular ED drug users among infertile test subjects showed modest improvements in sperm motility and shape, the drugs appeared to have little, if any, effect on sperm volume or sperm concentration.

PDE5 inhibitors such as Viagra, Levitra, and Cialis get their family name from their ability to temporarily sideline an enzyme known as phosphodiesterase-5.  By holding the PDE5 enzyme at bay, these drugs allow the unfettered workings of a substance known as cyclic guanosine monophosphate, or cGMP. This substance plays a key role in erectile function by signaling the smooth muscle tissue that lines artery walls to relax, thus expanding blood flow and facilitating erection.

Role of cGMP in Sperm Motility

As it turns out, cGMP also seems to play a major role in the movement of sperm, based on the findings of a study published in the January 2011 issue of “Reproduction,” the official journal of the U.K.-based Society for Reproduction and Fertility. Conducted by a team of Italian and Iranian research scientists, that study explored the role of nitric oxide and the subsequent reactions that nitric oxide activates on sperm motility.

Researchers said their findings led them to conclude that nitric oxide stimulates sperm motility through the activation of soluble guanylate cyclase, the subsequent synthesis of cGMP, and the activation of cGMP-dependent protein kinases. The findings from the study offer a link between cGMP and sperm motility that seems to have been elaborated upon in the more recent research review.

Conclusive Evidence Elusive

Conclusive evidence of PDE5 inhibitors’ beneficial effects on male fertility remains elusive, however. The conclusion drawn by the Chinese review of scientific literature contradicts earlier findings from a study conducted more than a decade earlier at Northern Ireland’s Queen’s University.

In an interview with NewScientist.com, David Glenn, a participant in that  Queen’s University study, said it concluded that sildenafil, the active ingredient in Viagra, seemed to trigger a premature activation of the sperm’s acrosome reaction. In a normal acrosome reaction, digestive enzymes embedded in the outer layer at the head of the sperm break down the outer wall of the female egg, thus allowing the sperm to penetrate and fertilize the egg.

Given this apparent contradiction about the effects of oral ED medications on male infertility, additional studies, preferably on a much larger scale, will have to be conducted to resolve some of the remaining questions on this issue.

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