Men who smoke or have smoked marijuana have significantly higher sperm counts than those who have never smoked it, according to a study published Tuesday in the journal Human Reproduction.
The research, led by Harvard scientists and conducted at the fertility clinic of Boston’s Massachusetts General Hospital, was based on samples from more than 600 men.
The finding is surprising; in recent years some studies have shown that marijuana smoking reduces sperm counts, in part because smoking is involved. Other studies have shown a link between cannabis smoking and improved sperm quality.
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No study has offered a biological explanation for its findings. The studies are based in part on questionnaires asking the subjects about their marijuana smoking (or nonsmoking) habits. Not always are personal or environmental variables taken into account such as genetics, climate, lifestyle or the different strains of marijuana consumed.
“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, an associate professor of nutrition and epidemiology at Harvard’s T.H. Chan School of Public Health. “Our results need to be interpreted with caution and they highlight the need to further study the health effects of marijuana use.”
For this study, researchers collected 1,143 semen samples from 662 men between 2000 and 2017. On average, the men were 36 years old, and most were white and college-educated. Also, 317 participants provided blood samples that were analyzed for reproductive hormones.
The questionnaire asked the men if they had ever smoked more than two joints or the equivalent amount of marijuana, 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 said they were current smokers.
The researchers found that men who had smoked marijuana had average sperm concentrations of 62.7 million sperm per milliliter of ejaculate, while for men who had never smoked cannabis the number was 45.4 million. Only 5 percent of marijuana smokers had sperm concentrations below 15 million per milliliter (the World Health Organization’s threshold for “normal” levels), compared with 12 percent of men who had never smoked marijuana.
The study also found that among marijuana smokers, greater use was associated with higher testosterone levels.
Still, the researchers warned about the study’s limitations. They noted that participants may have underreported marijuana use given its status as an illegal drug for most of the study period. They also stressed that they did not know to what extent their findings might apply to men in the general population, because the study population consisted of men in couples seeking fertility treatment.
According to Dr. Feiby Nassan, the lead author of the study and a postdoctoral research fellow at Harvard, the findings contradicted the researchers’ hypothesis but are consistent with two different interpretations. The first is 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,” Nassan said.
“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.”
An estimated 16.5 percent of American adults use marijuana, and in recent years there has been growing support to legalize its use despite the perception among many that it poses a number of health risks.