Men who have smoked marijuana at some point in their lives had significantly higher concentrations of sperm compared with men who have never smoked weed, according to a new research by Harvard University.
The study looked at men who were seeking help at fertility clinics, not the general population.
But the findings, detailed in the Human Reproduction journal, stands in contrast to previous studies that suggested smoking weed can negatively affect male reproductive health.
"These unexpected findings highlight how little we know about the reproductive health effects of marijuana, and health effects of marijuana in general," said Jorge Chavarro, Associate Professor at Harvard T.H. Chan School of Public Health in the US.
"Our results need to be interpreted with caution and they highlight the need to further study the health effects of marijuana use," Chavarro said.
In the new study, the researchers hypothesised that marijuana smoking would be associated with worse semen quality. They found no significant difference in sperm concentrations between current and former marijuana smokers.
In addition, among marijuana smokers, greater use was associated with higher serum testosterone levels.
It could be because "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, Postdoctoral Research Fellow at Harvard.
"An equally plausible interpretation is that our findings could reflect that men with higher testosterone levels are more likely to engage in risk-seeking behaviours, including smoking marijuana," Nassan explained.
The team collected 1,143 semen samples from 662 men aged 36 assessed for infertility.
Only 5 per cent of marijuana smokers had sperm concentrations below 15 million/ml compared with 12 per cent of men who never smoked marijuana.
Understanding the health effects associated with marijuana use is important given the growing perception that it poses few health hazards, the study suggested.