Led by researchers at Harvard T.H. Chan School of Public Health, the new study was carried out at the Fertility Clinic at Massachusetts General Hospital where the team collected 1,143 semen samples from 662 men in couples seeking treatment, with an average age of 36 between 2000 and 2017.
Blood samples were also collected from 317 of the participants in order to analyze the levels of reproductive hormones.
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.
As the researchers hypothesized that marijuana smoking would be associated with poorer semen quality, they were surprised to find that men who had smoked marijuana at some point in their lives actually had a higher average sperm concentration – 62.7 million sperm per ml of ejaculate – than men who had never smoked, who had an average concentration of 45.4 million sperm per ml of ejaculate.
The findings, published in the journal Human Reproduction, also showed that only 5% of marijuana smokers had sperm concentrations below 15 million/mL, which is the World Health Organization’s threshold for “normal” levels, compared with 12% of the non-smokers.
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.
“Our findings were contrary to what we initially hypothesized. 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,” commented Feiby Nassan, lead author of the study.
“An equally plausible interpretation is that our findings could reflect the fact that men with higher testosterone levels are more likely to engage in risk-seeking behaviours, including smoking marijuana.”
However, the authors also advised caution when interpreting the findings, saying that the study does have limitations including the possibility of participants underreporting marijuana use because of its illegality during much of the period, as well as highlighting that it’s unclear how much the results would extend to the general male population.
They further note that although previous studies have suggested that marijuana may have a negative effect on male reproductive health, they have mostly been animal studies or focused on men with histories of drug abuse.
“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 Jorge Chavarro, “Our results need to be interpreted with caution and they highlight the need to further study the health effects of marijuana use.”