The settled science on marijuana and its effects on male fertility just went up in smoke, as research from boffins at Harvard indicates that, contrary to popular belief, weed may increase sperm concentration, not reduce it.

Arguably the most commonly used drug worldwide, marijuana legalization across the world has led many academics and health experts to revisit the drug’s impact on human health.

The latest, long-term study conducted by Harvard University, involving semen samples from 662 men over the course of 17 years, showed that those who said they had smoked marijuana at least once in their lives had higher concentrations of sperm than those who abstained.

Some 55 percent of respondents had, at one point, smoked the drug before, 11 percent of whom were current marijuana smokers.

“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,”said co-author Jorge Chavarro, an expert in nutrition and epidemiology at Harvard University.

“Our results need to be interpreted with caution and they highlight the need to further study the health effects of marijuana use.”

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A total of 317 of the participants also provided blood samples that were analyzed for reproductive hormones. Among the marijuana-smoker group, researchers noted higher serum testosterone levels.

The Harvard study does come with several caveats, however: the men, mostly white, college educated, and with a mean age of 36, were asked if they had smoked more than two joints in their lives and were also seeking treatment at a fertility center. Many may not have wished to disclose whether they were taking any drugs that could potentially impact the fertility treatments they were undergoing.

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In addition, the demographics of the study do not accurately represent the wider US, let alone the world.

Also, while marijuana does interact with the endocannabinoid system, there is no clear causal link, as there are many other control factors that could explain the findings, argue the researchers.

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“An equally plausible interpretation is that our findings could reflect the fact that men with higher testosterone levels are more likely to engage in risk-seeking behaviours, including smoking marijuana,” said Feiby Nassan, a researcher in environmental health and nutrition at Harvard.

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So, while the university’s researchers aren’t exactly telling men to go out and spark up a bong, at least for fertility purposes, it apparently won’t harm their chances of producing offspring either.

For context, a 2015 study found that Danish men who smoked more than once a week had lower sperm counts; while another 2018 study found that the psychoactive ingredient in marijuana, THC, can alter sperm DNA and structure – not ideal when trying to conceive a baby.

The research was published in the journal Human Reproduction.

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