The men included in the research - mostly white, educated, with an average age of 36 - were all part of couples seeking help with conception from the Massachusetts General Hospital Fertility Centre between 2000 and 2017, and most of the men had normal sperm counts.
The 365 men who reported having ever smoked pot had "significantly higher sperm concentration" - precisely 63 million sperm per milliliter of semen - compared to the 297 who have never taken a hit, which averaged 45 million/mL.
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 risky than opioids by the USA government remains a roadblock to that research, incidentally).
"These unexpected findings from our study highlight that we know too little about the reproductive health effects of cannabis and, in fact, of the health effects in general, to make strong statements about the impact of cannabis on health, with the possible exception of mental health", said lead researcher Jorge Chavarro, an associate professor of nutrition and epidemiology at Harvard, in a statement.
However, the authors also advised caution when interpreting the findings, saying that the study does have limitations including the possibility of participants underreporting marijuana use because of its illegality during much of the period, as well as highlighting that it's unclear how much the results would extend to the general male population. 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).
The men who admitted to smoking marijuana at some point in their life seemed to have higher concentrations of sperm than those who had never had a puff. Along with their medical history, the men were also asked if they had ever used cannabis and, if they had, whether they were still using it.
On the other hand the association could have nothing to do with the effects of cannabis.
She said: "Our findings were contrary to what we hypothesised at the start of the study". But maybe men with high testosterone are just more likely to do drugs.
Numerous older studies had focused on animal models or had examined men with histories of drug abuse. The men in the study reported smoking an average of two joints per week.
Experts believe there may be a link between moderate cannabis use and how that can benefit the male's reproductive system.
In fact, this group was more than twice as likely to drop below the World Health Organisation's threshold for "normal" sperm levels.