Occasional marijuana use may not be as bad for male fertility as was once thought. Current and former marijuana smokers seen at a fertility clinic in Massachusetts had much higher sperm counts than men who had never smoked marijuana.

“These unexpected findings …highlight that we know too little about the reproductive health effects of cannabis and, in fact, of the health effects in general, to make strong statements about the impact of cannabis on health, with the possible exception of mental health,” said Jorge Chavarro, one of the study's co-authors.

Previous studies have suggested that marijuana has an adverse effect on sperm production, especially among heavy users, but the effect of moderate usage has been less clear.

The Harvard T.H. Chan School of Public Health researchers analyzed information on over 650 men who visited the Massachusetts General Hospital Fertility Center between 2000 and 2017. The men had provided details about frequency and duration of marijuana use. Blood and semen samples were also taken.

The 365 men who had smoked marijuana had a substantially higher average sperm count than the 297 men who had never smoked — 63 million sperm per milliliter of semen as compared to 45 million, after adjusting for factors known to affect sperm concentration, such as age, alcohol and cocaine use.

The average amount of marijuana smoked by people in the study was just over two joints a week. Those who smoked more marijuana also had slightly higher blood testosterone levels.

The findings should be treated with caution, the researchers warn. Not only is sperm concentration a poor predictor of fertility, it's also not clear if their findings are generalizable to the general public. But the results certainly were not what they were expecting.

Previous studies have suggested that marijuana has an adverse effect on sperm production, especially among heavy users. However, the effect of moderate usage has been less clear.

Among the possibilities raised by the current study are that low levels of marijuana use may have a beneficial effect on sperm count, while higher levels of use have a deleterious effect, what scientists call a U-shaped relationship. It's also possible that the observed differences in sperm counts are not actually caused by the differences in marijuana use.

The researchers hope that further studies will shed more light on the topic. According to Chavarro, one thing the study does seem to make clear is that “We know a lot less than we think we know.”

The study is published in Human Reproduction.