It may be that some marijuana stimulates sperm production, a ratio that reverts at higher doses.
When Dr. Jorge Chavarro's team began investigating the effects of marijuana on sperm, they had every reason to believe that marijuana would be detrimental to "testicular function," because other studies had said yes.
Instead, they found the opposite.
Men who have ever smoked marijuana had significantly higher sperm concentrations and sperm counts, as well as higher testosterone levels compared to men who never smoked marijuana.
Overall, the sperm of past and current marijuana users appeared to be of superior quality.
"These results are not consistent with a deleterious role of marijuana smoking in testicular function, as initially hypothesized," report Chavarro and colleagues this week in the journal Human Reproduction.
We know much less than we think we know
Earlier studies – most of them in rats, but some in human men as well – linked heavy marijuana use with a drop in sperm production. The men in the new study, however, smoked on average two joints per week.
Chavarro can not fully explain his team's unexpected findings, although he has a few chances. It may be that men with higher circulating concentrations of testosterone are also more likely to smoke marijuana and engage in other "risk behaviors," he and his co-authors postulate.
But it may also be true that a little marijuana increases sperm production, a ratio that reverts at higher doses, just as the incidence of heart disease is lower in people who drink moderately compared to those who do not.
According to Chavarro, this is clear: legal access to marijuana is moving faster than science about the effects of marijuana on the body. "We know much less than we think we know," said associate professor of nutrition and epidemiology at Harvard University.
A widely publicized study in 2014 involving nearly 2,000 British men – the world's largest study exploring how common lifestyle factors influence sperm morphology (the size and shape of sperm) – found that men under 30 with less than 4% of normal sperm twice as likely to have used cannabis within the past three months. No similar association was found with body mass index, underwear type, smoking, alcohol consumption or history of mumps – although researchers found that sperm size and shape were worse in the ejaculated samples in the summer months.
Sperm with morphological problems tend to be bad swimmers, crawling or crashing headlong into the walls of the female reproductive tract in their frenzied dive to fertilize an egg.
For the new study, the researchers collected 1,143 semen samples from 662 men between the years 2000 and 2017. The men were enrolled in the fertility center of Massachusetts General Hospital; 317 of them also provided blood samples that were analyzed for reproductive hormones.
The men were, on average, 36 years old, mostly white and mostly university graduates.
Just over half (55 percent) reported having smoked marijuana at some point. Of these, 44 percent were ex-smokers and 11 percent were current.
Men who smoked marijuana had an average sperm concentration of 62.7 million sperm per milliliter of ejaculate, compared with 45.4 million per mL in men who had never used marijuana.
There were no significant differences in sperm concentrations between current and past marijuana smokers.
A similar pattern was observed for total sperm count.
Only five percent of marijuana smokers had sperm concentrations below 15 million / mL, the World Health Organization threshold for "normal" levels, compared with 12 percent of men who never smoked marijuana.
Marijuana smokers also had lower concentrations of follicle-stimulating hormone, or FSH, a hormone produced by the pituitary gland. When the testes are having problems producing sperm, the pituitary gland makes up for more FSH.
It is possible that not all men were informed about weed use due to illegal drug status during most of the study (Massachusetts voted to legalize recreational marijuana in 2016). And the results may not be generalizable for all men because men in this study were enrolled in a fertility center (although the association between marijuana and better function of the testes occurred after researchers restricted their analyzes to men without a diagnosis of male factor). infertility.)
Who knows – it may be that marijuana really is positive for sperm production
Marijuana users were smoking relatively modest amounts of marijuana, two to three joints per week, on average.
Animal studies suggest that endogenous or natural cannabinoids play a critical role in sperm breeding. It is possible that THC in the pot could increase spermatogenesis, but after a certain level, the effect would reverse, said Chavarro.
Other researchers recently warned about the drop in sperm count among Western men. However, Chavarro said his team's article "does not mean that using more marijuana will increase sperm count, testosterone or masculinity."
The paper may be an outlier. "But who knows – it could be that marijuana is really positive for sperm production, and we're missing the answer," Chavarro said.
"The problem is that we can not say which of these two interpretations is correct."