For this recent study, researchers analysed health surveys from over 650 men who sought fertility treatment at the Massachusetts General Hospital Fertility Center between 2000 and 2017, focusing on each man's semen volume, sperm count, sperm motility, and hormone concentrations.
In measuring the survey results with the semen sample analysis, the scientists found that those using marijuana tended to have higher sperm concentrations and counts than those who never smoked it.
Participants were also asked to complete a questionnaire about their marijuana use, including if they had ever smoked more than two joints - or the equivalent amount of marijuana in their life - and if they were current marijuana smokers. However, the new study showed that men who smoked weed had an average sperm concentration of 62.7 million sperm per millilitre (million/mL). That's a lot.
There was also no significant difference in sperm concentrations between current and former marijuana smokers, and greater use of marijuana among marijuana smokers was associated with higher testosterone levels in the blood.
"These findings do not mean that using marijuana will increase sperm counts".
United States lead researcher Dr Jorge Chavarro said: "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.
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Secondly, the study is a great opportunity to spark interest on investigating the health effects of marijuana particularly with the backdrop of increasing legalization of recreational use in the USA coupled with a greater perception that marijuana poses no health risks".
She also explained they are "consistent with two different interpretations" - the first being that low levels of marijuana use could benefit sperm production.
One explanation is that the men in the study who smoked marijuana already had higher sperm counts than those who did not. Second, because the study's sampling wasn't diverse - 88% of the men were Caucasian, 84% were college educated, and the average age was 36.3 years old - the results may not apply to the general population. Allan Pacey, professor of andrology at the University of Sheffield, U.K., authored a 2014 study suggesting that using cannabis can impact the size and shape of sperm, and in turn male fertility.
On the other hand the association could have nothing to do with the effects of cannabis.
It's possible not all the men came clean about their use of weed, given the drug's illegal status during most of the study (Massachusetts voted to legalize recreational marijuana in 2016). Another reason, according to the researchers, is that men may be more likely to engage in "risk-seeing activities", like smoking marijuana, if they have higher testosterone concentrations- something that aids in fertility.
Vij pointed out that both marijuana users and nonusers in the study had normal sperm counts, on average.
Only 5% of cannabis users had sperm counts below 15 million/mL, the World Health Organisation's threshold for "normal" levels, compared with 12% of men who had never smoked cannabis.