Then Dr. Jorge Chavarro's team began to investigate the effects of marijuana on semen, they had every reason to believe that weed products would be detrimental to "testicular function" because other studies had said that was so.
Instead, they found the opposite.
Men who had ever smoked marijuana had significantly higher sperm concentrations and sperm, as well as higher testosterone levels compared to men who had never smoked weeds.
Overall, sperm cells in past and present marijuana users seemed to be of the highest quality.
"These results are not consistent with a harmful role for marijuana smoking on testicular function as originally hypothesized," Chavarro and his colleagues report this week in the journal Human Reproduction.
We know much less than we think we know
Previous studies – the majority of them in rats, but also a couple in men – have associated heavy pot use with a decrease in sperm production. The men in the new study averaged two joints a week.
Chavarro cannot fully explain his team's unexpected findings even though he has some hypotheses. It may be that men with higher circulating testosterone concentrations are also more likely to smoke pan and engage in other "risk-taking" behaviors, he and his co-authors postulate.
But it may also be true that a little bit of pot increases sperm production, a ratio that reverses at higher doses, much the way in which the incidence of heart disease is lower in moderate drivers compared to non-drinkers.
According to Chavarro, this is very clear: legal access to the pot is moving faster than the science of weed effects on the body. "We know much less than we think we know," said Associate Professor of Nutrition and Epidemiology at Harvard University.
A widely used 2014 study of almost 2,000 British men – the world's largest study to investigate how common lifestyle factors affect sperm morphology (sperm size and shape) – found that men under 30 with less than four percent common sperm were almost twice as likely to have used cannabis in the previous three months. No similar associations were found with body mass index, type of underwear, smoking, alcohol consumption or have had a history of hats – although the researchers found that the size and shape of the seat were worse in samples ejaculated during the summer months.
Sperm with morphology problems tend to be poor swimmers, crawl or collide head on the walls of the female reproductive tract in their hectic swim to fertilize an egg.
For the new study, researchers collected 1,143 semen samples from 662 men between 2000 and 2017. The men were enrolled at Massachusetts General Hospital's fertility center; 317 of them also supplied blood samples which were analyzed for reproductive hormones.
The men were on average 36 years, most white and mostly university graduates.
Just over half (55 percent) reported having smoked marijuana at some point. Of these, 44 percent were past smokers and 11 percent were present.
Men who had smoked pan had average sperm concentrations of 62.7 million sperm per day. Ml ejaculate compared to 45.4 million / ml in men who had never used marijuana.
There were no significant differences in sperm concentrations between current and previous marijuana smokers.
A similar pattern was seen for total sperm.
Only five percent of marijuana smokers had semen concentrations below 15 million / ml, the World Health Organization's "normal" levels threshold, compared to 12 percent of men who had never smoked pot.
Marijuana smokers also had lower concentrations of follicle stimulating hormone or FSH, a hormone produced by the pituitary gland. When the testicles have trouble producing semen, the pituitary gland compensates by producing more FSH.
It is not possible that all men were cleansed of their use of weeds, as the substance became illegal during most of the study (Massachusetts voted to legalize recreational marijuana in 2016). And the results may not be generalized to all men because the men in this study were enrolled from a fertility center (although the relationship between the pot and the better functioning of the testes kept by the researchers limited their analysis to men without the diagnosis of male factor infertility.)
Who knows – marijuana may actually be positive for sperm production
Marijuana users smoke relatively modest amounts of marijuana, two to three times a week on average.
Studies in animals suggest endogenous or natural cannabinoids play a crucial role in sperm formation. It is possible that THC from the pot could improve spermatogenesis, but after a certain level the effect would be reversed, Chavarro said.
Other researchers have recently warned to sense sperm among western men. But Chavarro said his team's paper "doesn't mean that the use of multiple marijuana will increase sperm counts, or testosterone or your masculinity."
The paper can be an outlier. "But who knows – marijuana may actually be positive for semen production, and we've got the wrong answer," Chavarro said.
"The problem is that we can't tell which of the two interpretations is right."