The results emerged from a study of 600 men who were studied by scientists in a fertility clinic. The men also answered lifestyle questions including whether they used marijuana or other drugs: 55 percent had smoked cannabis at least once; 44 percent were past users; and 11 percent current. It's also possible that, as some research has suggested, heavy or early cannabis use is really what can harm sperm, while the occasional puff as an adult might provide a boost to fertility. "An equally plausible interpretation is that our findings could reflect the fact that men with higher testosterone levels are more likely to engage in risk-seeking behaviours - including drug use - and that the relations we see between cannabis smoking, sperm counts and testosterone levels are because men with higher testosterone, within normal levels, have higher sperm counts and are more likely to smoke cannabis". They found no significant difference in sperm concentrations between current and former marijuana smokers. This system sends signals to the brain and these signals may play a role in fertility, they explain. Male cannabis users were also less likely to have unhealthy sperm counts than teetotalers.
The study shows that the men who reported smoking marijuana had an average sperm count of 63 million sperm per milliliter of semen, in comparison to 45 million sperm per milliliter of semen among those who have never used the drug.
Dr Jorge Chavarro said the findings were unexpected - and highlight how little is known "about the reproductive health effects of marijuana, and in fact the health effects of marijuana in general". In December 2018, INSIDER reported on a small study from Duke University that suggested marijuana use could be linked to lower sperm concentrations, a factor that can affect a man's fertility.
Regardless of why this connection exists, the authors say it's important to study further-especially since an estimated 16.5% of US adults use marijuana and that recreational use of the drug will likely be legalized in more states in the coming months and years.
The first author of the study, Dr Feiby Nassan, a postdoctoral research fellow at the Harvard T.H. Chan School of Public Health, said: "Our findings were contrary to what we hypothesised at the start of the study". For men who smoke marijuana and are planning on having children, the advice keeps getting more confusing.
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Additionally, the hormone levels of these men were also different. Both of those numbers are still considered healthy; the World Health Organization's threshold for "normal" levels is all the way down at 15 million/ml.
"These findings do not mean that using marijuana will increase sperm counts".
"What this study shows is pot smoking doesn't make you automatically infertile, but there have been studies on pot smoking for many years and there are a good number of studies out there that indeed have reported adverse effects on semen in pot smokers", Gleicher said.
However, the effects of more moderate marijuana use on sperm counts among men is less clear.
Fifty-five percent of the men said they'd smoked pot at some point, with 44 percent saying they were past smokers and 11 percent reporting they now toke. In addition, a study from a year ago found that marijuana's active ingredient, THC, can change the structure and development of a sperm's DNA.