"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, lead author of the study and a postdoctoral research fellow at the Harvard T.H. Chan School.
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. The men answered survey questions about how often they smoked marijuana or used other drugs, and they also provided sperm and blood samples.
"An equally important limitation is the fact that most of the data were collected while cannabis was illegal in MA, so it is hard to know to what extent men may have under-reported use of cannabis because of social stigma or potential consequences related to insurance coverage for infertility services", Chavarro told Fox News.
However, researchers also observed that people who stopped smoking tended to have slightly higher sperm counts than current pot smokers. Allan Pacey, professor of andrology at the University of Sheffield, U.K., authored a 2014 study suggesting that using cannabis can impact the size and shape of sperm, and in turn male fertility.
Dr Chavarro said: "It is well-documented that within normal ranges, high testosterone levels are associated with greater engagement in risk-seeking behaviors, including drug use".
While cocaine use was associated with a higher estimated proportion - 12 per cent having sperm concentration below World Health Organization reference values.
Lastly, there's also the reality that many men in this study might have been reluctant to admit they've smoked pot.
They expected cannabis to have a detrimental effect on sperm count and fertility.
Regardless of why this connection exists, the authors say it's important to study further-especially since an estimated 16.5% of US adults use marijuana and that recreational use of the drug will likely be legalized in more states in the coming months and years.
"There could be a non-causal explanation, such as the effect of the male hormone testosterone on both sperm count and risk-taking behaviour such as smoking cannabis", said United States lead researcher Dr Jorge Chavarro, from the Harvard TH Chan School of Public Health in Boston. For men who smoke marijuana and are planning on having children, the advice keeps getting more confusing. Meanwhile, 12 percent of non-cannabis consumers tested below that threshold for a normal sperm count.
The jury may still be out on whether marijuana use has an effect on a couple's ability to get pregnant, but it's worth keeping all of this research in mind if you're hoping to start a family anytime soon.
For now, there's just not enough evidence to make any conclusions about the use of marijuana on male sperm.
Experts looked at the effect of smoking an average of two joints a week among 662 sub-fertile men in Boston, Massachusetts.
However, the effects of more moderate marijuana use on sperm counts among men is less clear.
The finding does not necessarily mean that smoking cannabis increases the chances of fatherhood, the study authors and other experts were quick to point out. It's possible that low levels of marijuana could boost sperm quality, they say, while higher levels and repeated use could have a negative effect. The research is published in the journal Human Reproduction.