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.
The study by Harvard T.H. Chan School of Public Health in Boston, USA, involved measuring the sperm count of more than 600 men from couples attending a fertility clinic.
In the new study, the researchers analyzed details from 662 men who were evaluated for infertility from 2000 to 2017 at the Massachusetts General Hospital Fertility Center.
A new Harvard study says that men who smoke pot have a higher sperm count than their non-smoking counterparts.
When Dr Chavarro compared all the surveys, he found the marijuana users had a common result of having a slightly higher sperm counts and concentration, compared to men who had never used it.
They expected cannabis to have a detrimental effect on sperm count and fertility. Those who had never smoked marijuana only had 45.4 million sperm per millilitre.
None of these limitations or considerations invalidate study's findings, it's just a sign, as Chavarro said, that we need to study cannabis more (the fact that cannabis is still considered more unsafe than opioids by the USA government remains a roadblock to that research, incidentally).
"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". The men gave sperm and blood samples, as well as answering questions about how often they smoked marijuana or used other drugs.
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There could be a non-causal explanation for the association, such as the effect of the male hormone testosterone on both sperm count and risk-taking behaviour such as smoking cannabis. They were also all college-educated white men, so the results can't be applied to the wider population.
The researchers have cautioned that there are limitations to the findings, as some of the participants may have under-reported their cannabis use.
On the other hand the association could have nothing to do with the effects of cannabis. "For example, 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".
She said: "Our findings were contrary to what we hypothesised at the start of the study".
Scientists analysed the sperm quality of regular cannabis smokers as well as non-smokers, with some unexpected results. Allan Pacey, professor of andrology at the University of Sheffield, U.K., authored a 2014 study suggesting that using cannabis can impact the size and shape of sperm, and in turn male fertility.
Chavarro also went on to point out that if the study proves anything definitive, it is that the link between marijuana and general health remains something of a grey area.
A normal sperm count is at least 15 million/mL, according to the World Health Organization.