Forget the mellow slacker image -- pot smoking might actually make men more potent.
Men who've smoked marijuana appear to have significantly higher sperm concentrations than those who've never given it a try, a new study reports.
There's also a potential link between pot use and testosterone, said senior researcher Dr. Jorge Chavarro. He's an associate professor of nutrition and epidemiology at the Harvard T.H. Chan School of Public Health in Boston.
"With increasing use of marijuana, there was a positive association with serum testosterone levels," Chavarro said. "More marijuana, higher testosterone levels."
Couples shouldn't start smoking pot to improve their chances of conception, however.
This was an observational study, and it's entirely possible that the link between pot and male fertility might run in the opposite direction, Chavarro said.
"We know that men with higher testosterone levels tend to engage in risk-tasking behaviors. The higher your testosterone, the more likely you are to do risky stuff," like frequent pot smoking, Chavarro said.
Chavarro and his team set out to study the possible effects of pot smoking on male reproduction by observing 662 men enrolled at the Massachusetts General Hospital Fertility Center in Boston between 2000 and 2017.
Semen samples were taken from all of the men, and 317 provided blood samples that were analyzed for reproductive hormones. The men also filled out a questionnaire about their marijuana use.
Fifty-five percent of the men said they'd smoked pot at some point, with 44 percent saying they were past smokers and 11 percent reporting they currently toke.
The researchers had hypothesized that marijuana use would be associated with lower semen quality, so they were surprised to learn that pot smokers had higher sperm concentrations.
Current and past marijuana users had average sperm concentrations of 62.7 million sperm per milliliter of ejaculate, while men who had never smoked marijuana had average concentrations of 45.4 million sperm per milliliter.
Only 5 percent of marijuana smokers had sperm concentrations below 15 million, the World Health Organization's threshold for "normal" levels. By comparison, 12 percent of men who never tried marijuana had below-normal sperm levels, the findings showed.
These findings were "unexpected and surprising," Chavarro said.
"We spent a couple of months doing analysis to figure out if we had done something wrong," he added. "As far as we can tell, this is what the data is showing. Now the question is: What does it mean?"
Animal studies have shown that cannabinoid receptors play a part in the male reproductive system, Chavarro said. It could be that marijuana complements or boosts the role played by cannabinoids that are naturally produced within the body.
Or it could just be that guys with higher testosterone levels are more likely to smoke pot, as part of an overall pattern of risk-taking, he suggested.
Dr. Norbert Gleicher, medical director and chief scientist at the Center for Human Reproduction in New York City, said "it's too preliminary" to say whether pot would help or harm a man's fertility.
"What this study shows is pot smoking doesn't make you automatically infertile, but there have been studies on pot smoking for many years and there are a good number of studies out there that indeed have reported adverse effects on semen in pot smokers," Gleicher said.
"It requires further investigation, and I would argue those investigations have become more urgent now because pot has become more legalized," he added.
And pot smokers might not necessarily have a better chance of conception just because their sperm concentrations are higher, said Dr. Jennifer Kawwass, medical director of the Emory Reproductive Center in Atlanta.
Although there are more sperm, their quality might be affected by marijuana use in ways that remain undetected.
"It is important to understand that the difference in sperm concentration between light smokers and never-smokers is not necessarily associated with a difference in fertility potential," Kawwass said.
The new study was published Feb. 6 in the journal Human Reproduction.
More information Harvard Health has more about fertility-friendly lifestyle choices.
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