Marijuana use may affect the size and shape of men's sperm

Smoking marijuana may affect the shape and quality of young men’s sperm, ultimately putting their fertility at risk, new research shows.

In a study published in the medical journal Human Reproduction, researchers analyzed many different lifestyle factors that could possibly have an impact on male infertility.  The study was the largest of its kind, examining how habits such as smoking and drinking alcohol affected men’s sperm size and shape – also known as sperm morphology.

According to the researchers, marijuana use was the only habit they studied to be strongly associated with abnormal sperm morphology.

“We weren’t really interested in [the cannabis angle] at all,” Dr. Alan Pacey, senior lecturer in andrology at the University of Sheffield in England, told  “We were interested in trying to best define the risks of sperm quality.  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.”

For their study, Pacey and his team recruited more than 2,000 men from 14 fertility clinics in England, giving them detailed questionnaires about their medical history and lifestyle habits.

For reference on proper sperm morphology, the researchers used guidelines published by the World Health Organization, which detail what normal sperm should look like and the exact dimensions normal sperm should be. Sperm with abnormal morphology are thought to swim less well than normal sperm, making it harder to reach a woman’s egg and fertilize it.

Reliable sperm morphology data was only available for 1,970 of the men recruited, and of this group, 1,652 men produced “normal” samples of sperm – meaning over 4 percent of their sperm was the right shape size.  The other 318 men had abnormal samples, meaning less than 4 percent of their sperm morphology was correct.

When cross analyzing the sperm samples with the men’s lifestyle habits and other factors, the researchers found only two strong correlations.  The men with abnormal sperm samples were nearly twice as likely to have ejaculated during the summer months (from June to August), and they were much more likely to have abnormal sperm if they had smoked cannabis in the three months prior to ejaculating.

Although their study didn’t examine why marijuana may impact sperm shape, some preliminary studies have 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 said.  “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.”

But Pacey said the study has other implications besides the possible link between marijuana and infertility.  The researchers didn’t find any associations between other common lifestyle factors, such as cigarette smoking and alcohol consumption – two habits that have long been associated male infertility.

“This suggests your body mass index, your age, your ethnicity, your previous medical history, and your alcohol consumption didn’t have any effect,” Pacey said.  “What happens when people are trying for a baby, they tend to become really healthy, they often go on crazy episodes of putting out things that are part of their everyday life, and the evidence to support that is actually pretty weak.”

Pacey noted for couples who are trying to have a baby, men may want to abstain from cannabis use for some time, as it usually takes up to three months to make a sperm.  Otherwise, the number one predictor of normal sperm morphology and male fertility is ultimately genetics.

“There are fewer risks than people think, and that makes sense to me,” Pacey said. “The single thing that affects fertility is how big your testicles are.  If you’re blessed with big testicles, you’ll produce more sperm.”