The researchers are continuing to study the effects of cannabis smoking in couples and how this could affect the outcomes of treatments for infertility. What's more, all the of the men were enrolled at a fertility clinic, meaning they were seeking medical help with their partners to try to get pregnant.
As reported by the Bloomberg, the investigators collected over 1,143 semen samples from 662 men who had sought for contraception help at the health facility between 2000 and 2017.
Just over 50 per cent of the men reported using cannabis; half of these men reported using it in the past, while 11 per cent said they were current users.
However, the effects of more moderate marijuana use on sperm counts among men is less clear.
The findings, published in the journal Human Reproduction, also showed that only 5% of marijuana smokers had sperm concentrations below 15 million/mL, which is the World Health Organization's threshold for "normal" levels, compared with 12% of the non-smokers.
Chavarro warned that the findings don't necessarily mean that smoking pot increases the chances of making a baby.
"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", said lead researcher Jorge Chavarro.
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'Our results need to be interpreted with caution and they highlight the need to further study the health effects of marijuana use'. The researchers also studied blood samples provided by 317 men, in order to quantify the levels of reproductive hormones. Second, because the study's sampling wasn't diverse - 88% of the men were Caucasian, 84% were college educated, and the average age was 36.3 years old - the results may not apply to the general population.
Dr Feiby Nassan, another member of the Harvard team, said: "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 smoking marijuana".
The participants in the new study self-reported their marijuana use, so the details they gave may not have been accurate.
However, the scientists pointed out that most studies have hitherto focused on heavy drug users or animals.
Scientists analysed the sperm quality of regular cannabis smokers as well as non-smokers, with some unexpected results.
Among men who had ever smoked marijuana, those who used it more often had higher testosterone levels than those who used it less often.
On the other hand the association could have nothing to do with the effects of cannabis. For men who smoke marijuana and are planning on having children, the advice keeps getting more confusing. 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. Don't take weed if you want to start a family.