The study, published in the journal Human Reproduction, looked at how a man's lifestyle affects his sperm morphology: the size and shape of sperm. Researchers collected data from 1,970 men who provided semen as part of a fertility assessment.
All of the lifestyle information was self-reported, and researchers made no attempt to confirm accuracy. Of those men, 318 produced abnormal sperm, where less than 4% of it was the correct size and shape (as defined by the World Health Organization). The remaining men's sperm had a higher percentage at a "normal" size and shape.
"Cannabis smoking was more common in those men who had sperm morphology less than 4%," Pacey said. "Cannabis affects one of the processes involved in determining size and shape. And we also know that the way cannabis is metabolized is different in fertile and infertile men."
The study found that men who had less than 4% normal sperm were typically under 30 years old, had used marijuana within three months of giving their sample and were twice as likely to have provided their sample during the summer.
Any of those factors could have influenced sperm morphology, but Pacey said "the only thing we found that was a risk that a man can do something about was cannabis."
Image: Smoke, via Shutterstock