Men who smoked marijuana at some point in their lives had significantly higher concentrations of sperm compared to men who never smoked marijuana, according to new research from Harvard University.
The study looked at men seeking help in fertility clinics, not the general population.
But the findings, detailed in the journal Human Reproduction, contrast with previous studies suggesting that smoking marijuana may adversely affect male reproductive health.
"These unexpected findings highlight how little we know about the effects of marijuana on reproductive health and the effects of marijuana on health in general," said Jorge Chavarro, associate professor at Harvard T.H. Chan School of Public Health in the USA.
"Our results need to be interpreted with caution and they highlight the need for further study on the effects of marijuana use on health," said Chavarro.
In the new study, the researchers assumed that marijuana use would be associated with poorer semen quality. They found no significant difference in sperm concentrations between current smokers and former marijuana smokers.
In addition, among marijuana smokers, increased use was associated with higher levels of serum testosterone.
It may 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 use," said Feiby Nassan, a postdoctoral researcher at Harvard.
"An equally plausible interpretation is that our findings may reflect that men with higher levels of testosterone are more likely to engage in risk-seeking behaviors, including smoking marijuana," Nassan explained.
The team collected 1,143 semen samples from 662 36-year-old men evaluated for infertility.
Only 5% of marijuana smokers had sperm concentrations below 15 million / ml, compared to 12% of men who never smoked marijuana.
Understanding the health effects associated with marijuana use is important, given the growing perception that this poses few health risks, the study suggests.