Harvard University researchers tested sperm of 600 men who were currently enrolled at fertility clinics and asked them about their previous drug taking, for the study published in the journal Human Reproduction.
Around half of men had tried, or currently used, cannabis and the researchers found just 5 per cent had clinically low sperm counts, compared to 12 per cent of those who had never tried it.
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But because of the way the study is designed, measuring patients past habits and current sperm count, they cannot tell what their sperm levels were before they tried the drug - and therefore prove whether it had any effect.
"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,” said Dr Feiby Nassan, who was one of the uthors of the study
Surprising findings in science should generally be treated with more scepticism until they are repeated in follow-up research and the existing medical literature on cannabis and fertility should give aspiring fathers pause before they reach for their bongs.
Professor Sheena Lewis, from Queen's University Belfast, said her lab had studied the effects of cannabis use at similar levels reported by the Harvard team.
“Their sperm quality plummeted,” she said, adding that the sperm became less mobile and less able to penetrate the wall of the egg.
“Worst of all, sperm counts dropped and the nurse cells - also known as sertoli - that help to make sperm disappeared irreversibly.”
Fertility expert Allan Pacey, professor of andrology at the University of Sheffield, said strict drug laws in many countries currently meant high-quality trials cannot be completed, and so the evidence base is still not great.
However, he said he was "unconvinced" by the prospect of cannabis being a pancea for male ferility problems.
“In my opinion, this should be avoided at all costs in any couples trying to start a family," Professor Pacey said.