Smoking Marijuana Appears to up Men's Sperm Count—to the Surprise of Scientists

Smoking cannabis could boost a man's sperm count, according to a study. However, experts have cautioned the results aren't a green light to light up green.

As well as higher sperm counts, researchers found men who had smoked marijuana at least once had higher concentrations of sperm and lower serum follicle-stimulating hormone (FSH) levels. High FSH levels are linked to fertility problems in men.

Since 2012, nine states (and Washington) have legalized the recreational use of marijuana, and attitudes towards the drug have relaxed. The authors of a study published in the journal Human Reproduction wrote of "a growing perception that marijuana poses few health hazards and with increased legalization and decriminalization of recreational marijuana use worldwide."

However, it's unclear whether using cannabis helps or hinders a man's chances of conceiving. Some studies indicate that using cannabis can harm the creation of sperm, while others suggest men who use the substance have higher levels of testosterone: the hormone involved in making the reproductive cells. With their paper, the scientists hoped to answer whether the drug negatively impacts sperm.

The team assessed data on 622 men, with an average age of 36, who visited Massachusetts General Hospital Fertility Center between 2000 and 2017. In total, the men gave 1,143 semen samples. The researchers also studied blood samples provided by 317 men, in order to quantify the levels of reproductive hormones. The men also answered lifestyle questions including whether they used marijuana or other drugs: 55 percent had smoked cannabis at least once; 44 percent were past users; and 11 percent current.

Researchers found participants who had smoked marijuana at least once had higher sperm concentration, counts, and lower FSH concentrations compared with those who had never used the drug. At the same time, they didn't spot a difference in the sperm concentrations of men who used marijuana at the time of the questionnaire and those who had used it in the past.

However the authors wrote that their findings may not relate to the general population, and said their study was limited by the fact they relied on the men reporting their use of cannabis accurately.

Jorge Chavarro, study author and associate professor of nutrition and epidemiology at Harvard Chan School, commented: "These unexpected findings highlight how little we know about the reproductive health effects of marijuana, and in fact of the health effects of marijuana in general."

Feiby Nassan, lead author of the study and a postdoctoral research fellow at Harvard Chan School, told Newsweek: "Because the endocannabinoid receptors (the ones responsive to marijuana) are found in many places in the body especially the reproductive system, we need to understand their role on our health, especially with increasing their legalization.

"These findings do not mean that using marijuana will increase sperm counts. Secondly, the study is a great opportunity to spark interest on investigating the health effects of marijuana particularly with the backdrop of increasing legalization of recreational use in the U.S. coupled with a greater perception that marijuana poses no health risks."

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Researchers have investigated whether using cannabis affects a man's fertility. Getty Images

Other experts in the field have questioned how robust the association is. Allan Pacey, professor of andrology at the University of Sheffield, U.K., authored a 2014 study suggesting that using cannabis can impact the size and shape of sperm, and in turn male fertility.

Whether smoking marijuana affects sperm quality and quantity or not is a controversial topic, he said, and too few studies have been conducted to provide a concrete answer. What's more, it's a tough association to study as cannabis is illegal in many countries.

"This paper does not help us to get any closer to the truth," he said. "For example, the observation that men in this study who had ever (but not currently) smoked marijuana had a higher sperm concentration compared to those who had never done so is intriguing.

"But, critically, this does not demonstrate cause and effect. For example, as the authors point out, men with higher sperm concentrations are likely to have more testosterone in their bodies and thus may be more likely to smoke marijuana because simply they are willing to take more risks."

He added it is unclear whether the men were asked to detail their smoking habits before or after they were told their sperm quality results.

"There is a danger that those with higher sperm counts may have 'bigged up' their drug taking in order to have appeared more macho. Whereas men with poor sperm may have wanted to keep quiet about it so as to avoid any accusations of poor health behavior," he said.

Pacey warned it does not "give support to any apparent fertility benefits of smoking marijuana. In my opinion, this should be avoided at all costs in any couples trying to start a family."

Dr. Lindsey A. Hines, of the Centre for Academic Mental Health at the University of Bristol, U.K., said: "When interpreting these findings, it's important to remember that we can't tell if cannabis is causing a higher sperm count, as no measures of sperm count were taken before the men had started using cannabis."

This article has been updated with comment from Feiby Nassan.