Having smoked at least two joints of marijuana at some point in their lives was associated with higher sperm counts in men, a new study has found.
However, the researchers and other experts urged caution in interpreting the findings. Previous studies have suggested that smoking cannabis has adverse effects on sperm.
'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,' said study author Dr Jorge Chavarro, at the Harvard TH Chan School of Public Health in Boston, Massachusetts. 'Our results need to be interpreted with caution and they highlight the need to further study the health effects of marijuana use.'
The researchers analysed semen samples from 662 men attending the fertility clinic at Massachusetts General Hospital between 2000-2017. The men were an average age of 36, mostly white and college-educated. Blood samples were also taken from 317 of the men to analyse for reproductive hormones.
About 44 percent of the men said that had smoked at least two joints (or the equivalent of) marijuana at some point in the past; 11 percent said they were current users.
Men who had 'ever' smoked marijuana had significantly higher sperm concentrations at 62.7 million sperm per millilitre of ejaculate, than those who had 'never' smoked. They had average sperm concentrations of 45.4 million per ml.
Only five percent of 'ever' smokers fell beneath the World Health Organisation's threshold of 15 million per ml for 'normal' sperm concentration, compared with 12 percent of 'never' smokers.
The blood tests showed that having smoked marijuana at some point was also associated with higher testosterone levels.
'Our findings were contrary to what we initially hypothesised,' said study first author, Dr Feiby Nassan. 'However, they are consistent with two different interpretations, the first being that 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.'
Another interpretation, she said, was 'that men with higher testosterone levels are more likely to engage in risk-seeking behaviours, including smoking marijuana'.
Professor Allan Pacey at the University of Sheffield, who was not involved in the study noted that while the results were 'intriguing' they did 'not demonstrate cause and effect'.
He said: 'I am not convinced that this paper moves us any further forward in this debate. Moreover, nor does it give support to any apparent fertility benefits of smoking marijuana. In my opinion, this should be avoided at all costs in any couples trying to start a family.'
Dr Lindsey Hines at the University of Bristol, pointed out that the study focused on men attending a fertility clinic. She said: 'If the cannabis-smoking men in this study were compared to men in the general population, we may not see these results.'
The study was published in Human Reproduction.