Smoking cannabis occasionally may improve men's fertility by stimulating sperm production, a study by Harvard University suggests.
Analysis of more than 662 men found that those who had never taken the drug were more than twice as likely to have sperm concentrations below a "normal" threshold.
Published in the journal Human Reproduction, the results have taken the scientific community by surprise because previous research pointed to a negative effect for cannabis on fertility, the Daily Telegraph UK reports.
However, the scientists pointed out that most studies have hitherto focused on heavy drug users or animals.
They believe improved fertility in moderate users may be explained by a boost to the endocannabinoid system, known to play a role in sperm levels, from smoking cannabis.
Dr Jorge Chavarro, from the Harvard T.H. Chan School of Public Health, 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."
or 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.
More than half - 55 per cent - of the men reported having smoked cannabis at some point.
Of those, 44 per cent said they had taken the drug in the past, and 11 per cent 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 per cent of cannabis users had sperm counts below 15 million/mL, the World Health Organisation's threshold for "normal" levels, compared with 12 per cent of men who had never smoked cannabis.
The authors cautioned that their study does not prove cannabis improves fertility.
They said it was possible that men with more testosterone and better fertility are more likely to try drugs.