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Men who have smoked marijuana at some point in their life had significantly higher concentrations of sperm when compared with men who have never smoked marijuana, according to new research led by Harvard T.H. Chan School of Public Health. The study, conducted in the Fertility Clinic at Massachusetts General Hospital, also found that there was no significant difference in sperm concentrations between current and former marijuana smokers.
“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 Jorge Chavarro, associate professor of nutrition and epidemiology at Harvard Chan School. “Our results need to be interpreted with caution and they highlight the need to further study the health effects of marijuana use.”
The study was published on Feb. 5, 2019 in Human Reproduction.
The researchers hypothesized that marijuana smoking would be associated with worse semen quality. Previous studies on marijuana have suggested that it is associated with negative effects on male reproductive health, but most of those studies had focused on animal models or on men with histories of drug abuse.
For this study, researchers collected 1,143 semen samples from 662 men between 2000 and 2017. To gather information on marijuana use among study participants, researchers used a self-reported questionnaire that asked the men a number of questions about their usage, 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.
Among the participants, 365, or 55 percent, reported having smoked marijuana at some point. Of those, 44 percent said they were past marijuana smokers and 11 percent classified themselves as current smokers.
Analysis of the semen samples showed that men who had smoked marijuana had average sperm concentrations of 62.7 million sperm per milliliter of ejaculate while men who had never smoked marijuana had average concentrations of 45.4 million sperm per milliliter of ejaculate. Only 5 percent of marijuana smokers had sperm concentrations below 15 million sperm per milliliter (the World Health Organization’s threshold for “normal” levels) compared with 12 percent of men who had never smoked marijuana.
The researchers cautioned that there are several potential limitations to the findings, including that participants may have underreported marijuana use. The researchers emphasized that they do not know to what extent these findings may apply to men in the general population as the study population consisted of subfertile men in couples seeking treatment at a fertility center.
“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,” said Feiby Nassan, lead author of the study and a postdoctoral research fellow at Harvard Chan School. “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 behaviors, including smoking marijuana.”
