Can smoking cannabis really improve your sperm count?

The unexpected finding that men with a history of cannabis smoking had higher sperm counts has been widely reported after it was published in a leading fertility journal.

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.

But does the drug really improve your chances of parenthood?

1. How could it do this?

   The authors suggest that this could be a result of cannabis on the body's endocannabinoid system which sends chemical messages in the brain and has been associated with regulating fertility.

   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.

2. What other answers might there be?

   The study also found the cannabis user participants had higher testosterone levels, the male sex hormone that plays a key role in behaviour and sperm production.

   "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

3. What do other studies show?

   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.”

4. So what should couples looking to conceive think?

   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.