Men who smoke or have smoked moderate amounts of marijuana have significantly higher sperm counts than men who never touched the drug, according to a study published Tuesday in the journal Human Reproduction. The study looked at men who were seeking help at a fertility clinic, not the general population.
Heavy use of marijuana has been linked with sperm production trouble, but the effect of restrained use had not been clear. Now the new study does that, also showing that indulging in "a few joints" now and then is correlated with higher testosterone levels.
Moderate use was an average of two joints a week.
How, why might marijuana be associated with higher sperm production? The study doesn't ask that, but co-author Feiby Nassan suggests two ideas: low levels of marijuana might boost sperm production because it affects the endocannabinoid system, which is known to play a role in fertility.
Or, he points out, men with higher testosterone levels are more likely to engage in risk-seeking behaviors, including smoking marijuana.
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The study was not a large one, and was done on men seeking fertility treatments. Harvard scientists checked 662 subfertile men who came to the Massachusetts General Hospital Fertility Center between 2000 and 2017. Their cannabis use was self-reported.
Of the subjects, they found that 365 men who had previously smoked cannabis in moderate amounts had significantly higher sperm concentration than the 297 men who had never smoked. Note that the scientists adjusted for factors known to affect sperm production, including age, abstinence time, smoking, and intake of coffee, alcohol and cocaine.
The moderate smokers also had higher testosterone counts. Note that this does not address cause and effect - nobody is saying that smoking weed causes higher testosterone or sperm production.
Other studies have shown that heavy marijuana use reduces sperm counts, possibly in part because of tobacco in the joints. Yet other studies have shown a link between cannabis smoking and improved sperm quality.
The study does not explain its findings.
“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, among whom 365 reported past or present marijuana use. On average, the men were 36 years old, and most were white and college-educated.
The questionnaire asked the men if they smoke more than two joints a week, and if they were current marijuana smokers.
The researchers found that men with moderate marijuana use averaged 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 paper says.
The study had limitations. Who knows whether the participants were honest about their marijuana use, it being illegal for most of the study period. Also, again, the study was done on men seeking fertility treatments, and the results might not apply to the general population.
About 17 percent of American adults are estimated to use marijuana.