A history of smoking cannabis has unexpectedly been linked to greater fertility in men.
The result came as a big surprise to scientists measuring the sperm counts of more than 600 men from couples attending a fertility clinic.
They expected cannabis to have a detrimental effect on sperm count and fertility.
Instead, those participants who admitted ever taking the drug turned out to have higher sperm counts than non-users.
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.
There could be a non-causal explanation for the association, such as the effect of the male hormone testosterone on both sperm count and risk-taking behaviour such as smoking cannabis.
US lead researcher Dr Jorge Chavarro, from the Harvard T.H. 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.
"Our results need to be interpreted with caution and they highlight the need to further study the health effects of marijuana use."
Previous research involving animal experiments or men with histories of drug abuse has suggested that cannabis impairs male reproductive health.
For the new study, investigators collected 1,143 semen samples from 662 men between 2000 and 2017.
On average the men were 36 years old, mostly white and college educated. All of them belonged to couples seeking help with conception from a fertility clinic.
Participants were asked to fill in questionnaires detailing their history of cannabis use.
More than half (55%) of the men reported having smoked cannabis at some point. Of those, 44% said they had taken the drug in the past, and 11% classified themselves as current users.
Analysis of the semen samples showed that men who had smoked marijuana had average sperm concentrations of 62.7 million sperm per millilitre (million/mL). Those who had never smoked a joint had an average count of 45.4 million/mL.
Only 5% of cannabis users had sperm counts below 15 million/mL, the World Health Organisations threshold for "normal" levels, compared with 12% of men who had never smoked cannabis.
In addition, greater cannabis use was linked to higher levels of the male hormone testosterone among smokers.
Writing in the journal Human Reproduction, the scientists said it was possible that low level exposure to cannabis might benefit sperm production in some way.
The endocannabinoid chemical messenger system in the brain, which is targeted by the drug, is known to play a role regulating fertility.
On the other hand the association could have nothing to do with the effects of cannabis.
Dr Feiby Nassan, another member of the Harvard team, said: "An equally plausible interpretation is that our findings could reflect the fact that men with higher testosterone levels are more likely to engage in risk-seeking behaviours, including smoking marijuana."
This view was echoed by leading British expert Allan Pacey, Professor of Andrology at the University of Sheffield.
He said: "As the authors point out, men with higher sperm concentrations are likely to have more testosterone in their bodies and thus may be more likely to smoke marijuana because simply they are willing to take more risks.
"In conclusion, I am not convinced that this paper moves us any further forward in this debate.
"Moreover, nor does it give support to any apparent fertility benefits of smoking marijuana.
"In my opinion, this should be avoided at all costs in any couples trying to start a family."
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