Men who smoke marijuana may have higher sperm counts than those who have never used the drug, a surprising new study finds.
The findings are "not consistent" with previous research, which has suggested that marijuana has a harmful effect on men's testicular function, the researchers said.
However, the study, published in the Feb. 6 issue of the journal Human Reproduction, doesn't mean men should start smoking pot to up their sperm counts.
The findings are far from conclusive, and more research is needed to understand whether smoking marijuana could indeed, at certain levels, have a positive effect on sperm production.
But the study does highlight how little researchers know about the effects of marijuana on reproductive health, study senior author Dr. Jorge Chavarro, an associate professor of nutrition and epidemiology at the Harvard T.H. Chan School of Public Health in Boston, said in a statement. "We know a lot less than we think we know." [25 Odd Facts About Marijuana]Marijuana and sperm
Previous studies had suggested that smoking marijuana may lower a man's sperm count, especially among heavy users. For example, in 2015, researchers from Denmark found that men who smoked marijuana more than once per week had sperm counts that were nearly 30 percent lower than those who didn't smoke marijuana, or those who used the drug less frequently.
However, the effects of more moderate marijuana use on sperm counts among men is less clear.
In the new study, the researchers analyzed information from 662 men who, along with their partners, were evaluated for infertility from 2000 to 2017 at the Massachusetts General Hospital Fertility Center. The men answered survey questions about how often they smoked marijuana or used other drugs, and they also provided sperm and blood samples.
Overall, a little over half of the men (55 percent) reported ever smoking marijuana in their lifetimes, and 11 percent said that they currently smoked marijuana.
The researchers found that men who reported ever having smoked marijuana had an average sperm concentration of 63 million sperm per milliliter of semen, compared with 45 million sperm per milliliter of semen among those who had never used marijuana. The findings held even after the researchers took into account some factors that could have affected sperm concentration, such as age, cigarette smoking and alcohol use.
What's more, only 5 percent of the marijuana smokers had lower-than-normal sperm concentrations — that is, lower than 15 million sperm per milliliter of semen. Among men who never smoked marijuana, 12 percent had lower-than-normal sperm concentrations.
Among men who had ever smoked marijuana, those who used it more often had higher testosterone levels than those who used it less often.
Interestingly, each additional year that had passed since a man last used marijuana was tied to a slight increase in sperm count.
"Our findings were contrary to what we hypothesized at the start of the study," study lead author Feiby Nassan, a postdoctoral research fellow at the Harvard T.H. Chan School of Public Health, said in the statement.
But the study can be interpreted in several ways. It may be that low or moderate levels of marijuana use have a beneficial effect on sperm production, but heavier use reverses this effect. Or, it could also be that men with higher testosterone levels are more likely to engage in "risky" behaviors such as drug use; and the researchers found the link between marijuana and sperm count "because men with higher testosterone, within normal levels, have higher sperm counts and are more likely to smoke cannabis," Nassan said.Jury still out
It's known that moderate- to heavy-use of tobacco or alcohol is tied to lower sperm counts, but whether marijuana has the same effect is up for debate, said Dr. Sarah Vij, a urologist at the Cleveland Clinic who was not involved with the study.
Vij said she applauded the study authors for looking at this question, since it is a topic that needs more research.
But the new study doesn't provide a conclusive answer. "Overall, the jury is still out on how marijuana impacts a man's fertility potential," Vij told Live Science.
Vij pointed out that both marijuana users and nonusers in the study had normal sperm counts, on average. So the study can't draw any conclusions about whether marijuana use is tied to better fertility.
In addition, it takes about three months for men to undergo a full cycle of sperm production to produce mature sperm. This means that using marijuana years ago "really should not have any impact at all on [a man's] current fertility state," Vij said.
And yet, the study still found that men who said they used marijuana at least a year ago had higher sperm counts than men who used it more recently. Vij said she wondered if "there's something that goes along with marijuana use" that's tied to better sperm production.
The researchers also noted that their study was conducted among men who visited a fertility clinic, and so the results may not necessarily apply to the general population. In addition, men in the study self-reported their marijuana use, and it's possible that some participants were not truthful about their marijuana use, due to the social stigma or illegal status of the drug in Massachusetts at the time the data was collected.