Researchers have made the unexpected discovery that cannabis smoking is linked to higher sperm counts and higher testosterone levels among moderate users than men who never smoke cannabis.
The study, published in Human Reproduction, included 662 sub-fertile men attending a US fertility centre between 2000 and 2017. It found men who had ever smoked cannabis had significantly higher sperm concentration than men who had never smoked it (P=.0003).
Men who smoked cannabis more often had testosterone levels an average of 8 ng/dL higher than those who used it less often. Only 5 per cent of cannabis smokers had estimated sperm concentrations below normal levels compared with 12 per cent of men who had never smoked cannabis.
Author, Dr Feiby Nassan, from the Harvard T.H. Chan School of Public Health, said the findings are consistent with a U-shaped relationship whereby low levels of cannabis use could benefit sperm production, but beneficial effects reverse at higher levels of use.
Alternatively, she said, the results may reflect the fact that men with higher testosterone levels are more likely to engage in risk-seeking behaviours, including drug use.
The authors caution that chance findings cannot be excluded.