It is known that cigarette smoking may contribute to a decrease in fertility for both men and women. In comparison, existing studies have not been able to determine how marijuana could affect the reproductive system.

Some findings leaned toward the hypothesis that smoking marijuana would most likely have an adverse effect on sperm production. This was also what Harvard researchers expected to find when conducting their latest study, published in the journal Human Reproduction on Feb. 5.  

Instead, they found that men who smoked marijuana at some point had higher sperm concentrations than non-users. The researchers said they examined the data more than once just to make sure there had not been a mistake.

"We spent a good two months redoing everything, making sure that there wasn’t any error in the data," Dr. Jorge Chavarro, co-author and professor at the Harvard T.H. Chan School of Public Health, told TIME. "We were very, very surprised about this."

Semen samples from 662 men — among whom, 365 men had smoked marijuana at least once in their lives — underwent analysis. Overall, the subset had higher sperm concentrations and counts than those who have never smoked it in their lives. Those who smoked it more often were also found to have higher testosterone levels than those who did so rarely.

"Our findings were contrary to what we initially hypothesized," said Feiby Nassan, the lead author of the study and a postdoctoral research fellow at Harvard.

"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," he explained in a press statement.

But this association does not mean that men should start lighting up in hopes of boosting their fertility. It is still not understood whether marijuana use had such an effect or if men with higher testosterone were more likely to engage in risky behaviors such as the use of cannabis.

Furthermore, the study was conducted when cannabis was illegal in Massachusetts. This means some of the participants may have chosen to hide their drug use or lie about the extent of use. Semen quality cannot be used as a completely accurate indicator of fertility either, the study authors acknowledged.

As we all know, the United States has seen a rise in the legalization and recreational use of marijuana recently. If there is anything to take away from the study, it serves as an important reminder that there is still a lot to discover about the effects of cannabis on the human body, be it beneficial or harmful.