For years, it’s been reported that marijuana negatively affects male fertility, lowering men’s sperm count, lessening the quantity of their seminal fluid, and causing their sperm to behave abnormally. All in all, the message was: the effect of pot on men’s sperm is not good, and may be one of the reasons contributing to a growing male fertility crisis.

But new research out of Harvard University’s T.H. Chan School of Public Health challenges this steady narrative, finding no significant differences in sperm concentrations between current and past marijuana users. In fact, men who had smoked marijuana at some point in life had higher sperm concentrations than men who had never lit up a joint, and more frequent marijuana use was linked to higher testosterone levels in the blood, suggesting — though by no means proving — more nuance to marijuana’s effect on male fertility.

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The results surprised the research team. “Our findings were contrary to what we initially hypothesized. However, they are consistent with two different interpretations, the first being that low levels of marijuana use could benefit sperm production because of its effect on the endocannabinoid system, which is known to play a role in fertility, but those benefits are lost with higher levels of marijuana consumption,” said Feiby Nassan, PhD, lead author of the study and a postdoctoral research fellow at Harvard’s Chan School. The endocannabinoid system is a complex system of naturally occurring cannabinoids and nervous system receptors, which participate in regulating a variety of physiological processes, including fertility, pregnancy, mood and memory. It is also the bodily system that responds to the use of cannabis, that is, marijuana.

“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 behaviors, including smoking marijuana,” Nassan said.

The study was not without limitations. It involved only 365 participants, with an average age of 36, 44 percent of whom said they had smoked marijuana in the past, 11 percent of whom described themselves as current marijuana smokers, and 45 percent of whom had never smoked pot. Men self-reported their marijuana intake — a popular but also somewhat unreliable research method, especially given the context of illegal activity. (Marijuana was illegal in the US state where the research took place for most of the study period.) And all were considered ‘subfertile’ and were seeking treatment with their partners at the Fertility Clinic at Massachusetts General Hospital.

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This means that “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 Jorge Chavarro, PhD, associate professor of nutrition and epidemiology at Harvard’s Chan School.

Previous research has shown tetrahydrocannabinol (THC), the active ingredient in marijuana, affects sperm quality, as well as quantity. THC appears to inhibit proper DNA function in sperm, though it is unclear whether these changes are passed on to offspring. And tobacco, which is smoked with some forms of marijuana, has been decisively linked to low quality sperm, including low count, poor movement, more DNA damage and abnormal shape

Ultimately, the study is evidence that more research into the effects of marijuana on male fertility is needed, in order to reconcile disparate findings. “Our results need to be interpreted with caution and they highlight the need to further study the health effects of marijuana use,” Chavarro said.