The team of researchers led by the Harvard T.H. Chan School of Public Health collected semen samples from 662 men between 2000 and 2017 with an average age of 36. These men were asked about their cannabis habits and previous drug-taking behavior. Although the study of moderate marijuana use on sperm count among men is still unclear or "less clear".
It also didn't seem to matter whether the men had smoked in the past, or if they were current smokers, the difference remained the same.
The endocannabinoid chemical messenger system in the brain, which is stimulated when cannabis is in the body, can play a role regulating fertility. Among men who had ever smoked cannabis, those who smoked it more often had testosterone levels an average of eight nanograms per decilitre (ng/dL) higher than those who used it less often. They found that those who had smoked pot have a higher sperm concentration per ejaculate than those who never had the substance at all.
New US research has revealed a surprising link between marijuana and a man's fertility, finding that men who have smoked the drug have a significantly higher concentration of sperm compared to men who have never smoked it.
Lead researcher Dr Jorge Chavarro, from the Harvard TH Chan School of Public Health in Boston, said: "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. Secondly, the study is a great opportunity to spark interest on investigating the health effects of marijuana particularly with the backdrop of increasing legalization of recreational use in the US coupled with a greater perception that marijuana poses no health risks".
Numerous older studies had focused on animal models or had examined men with histories of drug abuse.
"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 USA lead researcher Dr Jorge Chavarro, from the Harvard TH Chan School of Public Health in Boston.
Among the participants, 365, or 55%, reported having smoked marijuana at some point. 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.
A sperm count of 15 million/mL is the the World Health Organisation's (WHO) threshold for "normal" levels. Researchers warned that heavy cannabis use could bring sperm levels back down to normal.
 "Marijuana smoking and markers of testicular function among men from a fertility centre", by Feiby L. Nassan et al.
"There seems to be a lot going on in terms of legalization, but not an equally fast pace in terms of knowing what the health effects of marijuana might be", Chavarro added.
However, the researchers say their results need to be treated cautiously as other factors may account for their findings.
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.
None of these limitations or considerations invalidate study's findings, it's just a sign, as Chavarro said, that we need to study cannabis more (the fact that cannabis is still considered more unsafe than opioids by the USA government remains a roadblock to that research, incidentally).
The findings appear in the journal Human Reproduction.
Fifty-five percent of the men said they'd smoked pot at some point, with 44 percent saying they were past smokers and 11 percent reporting they now toke.