Around 40% of infertility cases are due to problems with the size, shape, motility, and/or count of the man’s sperm. A recent study seeking to explore lifestyle factors that may influence sperm morphology discovered that young men who smoked marijuana were more likely lower amounts of normally-shaped sperm. The study was led by Allan Pacey of the University of Sheffield and the paper was published in the journal Human Reproduction.

Pacey and his team went to 14 fertility clinics in the UK and collected samples from 2,249 men of at least 18 years of age. 1,970 of these samples had reliable sperm morphology results. A man should have at least 40 million sperm in every 2 cc of ejaculate, and if at least 4% of them are the correct size and shape, his sperm morphology is considered normal. The researchers found that 318 of the men in the study were below normal. The remaining 1,652 men with normal morphology were considered the control group.

The study participants were then asked to self-report on certain lifestyle factors, such as smoking cigarettes, drinking alcohol, and using recreational drugs. The researchers also took note of the time of year the samples were donated, as heat can affect sperm morphology (which is why the testicles are located outside of the body).

The men younger than 30 whose normal sperm morphology was below 4% were more likely to have smoked marijuana during the three months before giving the sample as the control group. The age of 30 is somewhat arbitrary; while research has shown sperm quality does decrease with age, there isn’t a definitive threshold for decreased fertility like there is with women. These men were also more likely to have made the sample during the summer, which may have affected the morphology as well. It takes 72 days for sperm to mature, so any exposure to heat or drugs during this time could very well affect the quality of the sperm. Further study will need to be completed, controlling for factors such as heat and trauma, in order to verify or exonerate marijuana as a causal agent.

“Our knowledge of factors that influence sperm size and shape is very limited, yet faced with a diagnosis of poor sperm morphology, many men are concerned to try and identify any factors in their lifestyle that could be causing this,” Pacey said in a press release. “It is therefore reassuring to find that there are very few identifiable risks, although our data suggests that cannabis users might be advised to stop using the drug if they are planning to try and start a family."

The researchers did not find a link with other factors such as smoking cigarettes or drinking, though anyone planning on having a baby should probably cut back on those as well.

This study builds off of previous research Pacey co-authored in 2012, when the team explored lifestyle factors that influenced sperm motility. That study also found few things that men could change in order to increase their motility. However, the study only looked at factors like smoking or drinking habits, and never analyzed the quality of the DNA carried by the sperm, which may have yielded different results.

Cannabis isn't the only factor found to influence sperm morphology. A companion paper published in the Journal of Occupational and Environmental Medicine described men working with chemicals like paint strippers or lead would also be more likely to have a lower percentage of normal sperm. Previously, it had been shown that men who routinely had exposure to organic solvents were more likely to have slower-swimming sperm.

[Header image “Smoking” by Chuck Grimmett via flickr, used in accordance with CC BY-SA 2.0]