One widely circulated 2014 study involving almost 2,000 British men - the world's largest study to explore how common lifestyle factors influence sperm morphology (the size and shape of sperm) - found that males under 30 with less than four-per-cent normal sperm were almost twice as likely to have used cannabis in the previous three months. However, a new study by the Harvard T.H. Chan School of Public Health in Boston, involving more than 600 men had surprising results.
The team at Harvard tested 662 men who were enrolled at the Massachusetts General Hospital Fertility Center between 2000 and 2017.
As the researchers hypothesized that marijuana smoking would be associated with poorer semen quality, they were surprised to find that men who had smoked marijuana at some point in their lives actually had a higher average sperm concentration - 62.7 million sperm per ml of ejaculate - than men who had never smoked, who had an average concentration of 45.4 million sperm per ml of ejaculate.
They reckoned that there could be a "non-casual explanation" to the effect of male testosterone on "sperm count and risk-taking behaviours".
On the other hand, 12% of those who had never smoked bhang had a sperm count below the normal levels.
"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", says co-author Jorge Chavarro, an expert in nutrition and epidemiology at Harvard University.
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. The men answered survey questions about how often they smoked marijuana or used other drugs, and they also provided sperm and blood samples.
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The study also found that among the men who had ever smoked pot, "those who smoked it more often had testosterone levels an average of eight nanograms per decilitre higher than those who used it less often".
The men were, on average, 36 years old, mostly white and mostly university-educated.
The researchers have cautioned that there are limitations to the findings, as some of the participants may have under-reported their cannabis use.
Scientists found there was no significant difference in sperm concentrations between current and former smokers.
Numerous older studies had focused on animal models or had examined men with histories of drug abuse. He said: 'As the authors point out, men with higher sperm concentrations are likely to have more testosterone in their bodies and thus may be more likely to smoke marijuana because simply they are willing to take more risks.
If you're a male looking to increase your chances of getting a woman pregnant, you may want to start smoking marijuana.
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.