A study suggests that smoking marijuana may cause sperm abnormality and put men's fertility at risk. In the largest study of its kind — published in the Human Reproduction journal — researchers looked at sperm samples of 1,970 men from various fertility clinics in the United Kingdom. The scientists examined how smoking and drinking habits, as well as other lifestyle factors, such as BMI, medical history and the type of underwear worn, affected sperm.
"We weren't really interested in [the cannabis angle] at all. We were interested in trying to best define the risks of sperm quality," says Alan Pacey, a senior lecturer in andrology at the University of Scheffield in England. "We recruited [a couple thousand] guys, who gave us a sperm sample and allowed us to investigate aspects of their lives....It was just one of the things we asked if they did; it was no more detailed than that."
The results of the study revealed that marijuana was the only habit studied that was strongly associated with abnormal sperm morphology—that is, sperm that were oddly shaped or sized.
Of the men studied, 1,652 produced "normal" samples of sperm, meaning that more than 4 percent of their sperm was the right shape and size. The remaining men's sperm was shown as "abnormal."
Researchers found that men were much more likely to have abnormal sperm if they smoked cannabis within three months prior to ejaculating. While the study didn't examine how smoking marijuana affects sperm shape, it revealed that the drug could alter the sperm production process.
"I do know there is some work in laboratory animals that suggests [marijuana] can affect the way the DNA in the sperm is packaged together, and that's significant," Pacey says. "When sperm are made, the DNA they maintain has to be packaged in the head very tightly, and when that process doesn't work properly, you get an abnormal sperm. So the cannabis is maybe interrupting that DNA folding."
The American Society for Reproductive Medicine suggests that recreational drugs, including anabolic steroids and marijuana, are associated with impaired sperm function, and it urges men who are trying to conceive to quit immediately.
Rebecca Sokol, president of the ASRM, says that the study confirms previous studies, but that the link between abnormal sperm and its function has not been proven because the study does not include enough cases. However, she advises that physicians should counsel their patients about relationships between lifestyle factors, abnormal sperm and fertility outcomes.
"This should include a discussion that the data are often inconclusive, but the motto 'everything in moderation' is a wise approach for the couple who is planning a pregnancy," Sokol says.