The study, conducted in the Fertility Clinic at Massachusetts General Hospital, also found that there was no significant difference in sperm concentrations between current and former 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". For example, in 2015, researchers from Denmark found that men who smoked marijuana a few times per week had sperm counts that were almost 30 percent lower than those who didn't smoke marijuana, or those who used the drug less frequently.
"An equally plausible interpretation is that our findings could reflect the fact that men with higher testosterone level are more likely to engage in risk-seeking behaviors, including smoking marijuana", said Feiby Nassan, an environmental health and nutrition researcher at Harvard.
"We spent a good two months redoing everything, making sure that there wasn't any error in the data", Cahvarro was quoted by Bloomberg.
Numerous older studies had focused on animal models or had examined men with histories of drug abuse.
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. It's also possible that, as some research has suggested, heavy or early cannabis use is really what can harm sperm, while the occasional puff as an adult might provide a boost to fertility. Since marijuana was still illegal in MA at that time, it's possible that many test subjects under-reported or even lied about their cannabis consumption during the trial.
"The relations we see between cannabis smoking, sperm counts and testosterone levels are because men with higher testosterone, within normal levels, have higher sperm counts and are more likely to smoke cannabis", she said.
Out of the group of participants, 55 percent reported having smoked marijuana at some point. "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". 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".
The results were surprising - and show how much more there is to learn about the link between marijuana use and fertility, the scientists said. About half of the men in the sample had also gotten a blood test, allowing the researchers to look at their levels of hormones such as testosterone.
Experts looked at the effect of smoking an average of two joints a week among 662 sub-fertile men in Boston, Massachusetts.
Only five per cent of cannabis smokers had estimated sperm concentrations below 15 million/mL - the World Health Organisation's threshold for "normal" levels. While he's not certain of the drug's impact on male fertility, he believes it's more likely that men with higher testosterone levels will be more inclined to use marijuana.
Prof Chavarro said: "First, we do not know to what extent these findings may apply to men in the general population".