One possible explanation could be that men who generally produce higher testosterone levels are more likely to use marijuana, rather than the implication that cannabis use itself affects sperm potency. The researchers also collected blood samples from 317 participants for the goal of analyzing reproductive hormones.
"There are men in the world who have never smoked a joint in their life and they have lots of children and there are others who smoke every day and never got their partner pregnant", said Morrison.
Just over 50 per cent of the men reported using cannabis; half of these men reported using it in the past, while 11 per cent said they were current users.
Only 5 per cent of marijuana smokers had sperm concentrations below 15 million/ml compared with 12 per cent of men who never smoked marijuana.
Professor Sheena Lewis from Queen's University Belfast said: 'In studies from our lab in Queens University, Belfast, we found exactly the opposite effects.
The men involved in this study reported their own marijuana use, answering questions about whether they currently smoked weed, how much they smoked weed now, or if they had ever done so in the past. The men who had smoked marijuana also had lower levels of follicle-stimulating hormone, which has been associated with a greater risk of infertility, according to the National Institutes of Health (NIH). This had been the case in past studies, although most of those were very small, used animal models, or focused on men with histories of drug abuse.
"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", explained study co-author Jorge Chavarro in a statement.
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".
But the researchers urged caution over their results and called for more studies.
The study shocks many due to past research, which claimed that marijuana has a harmful side effect of lowering and killing off sperm count along with decreasing the testicular function. Although the study of moderate marijuana use on sperm count among men is still unclear or "less clear".
Although men who had smoked pot at some point in their lives had higher sperm numbers, the study did not look at cause and effect. Second, since semen quality is a poor predictor of fertility, these findings should not be interpreted as implying better fertility with cannabis use.
Given the illegal status of marijuana in some places and its social stigma, the researchers also have reason to believe some of the participants they studied underreported their marijuana use in the surveys.
"We know that men with higher testosterone levels tend to engage in risk-tasking behaviors".
Previous studies - the majority of them in rats, but a few in human males as well - have linked heavy pot use with a slump in sperm production. First, participants were only asked about smoking marijuana, not consuming it in other forms, like an edible.