It's long been an urban legend that cannabis consumption can result in male infertility. Now a new study published in the journal Human Reproduction reports that pot use – among other factors – may cause abnormal sperm quality.

Researchers from Sheffield and Manchester universities in the UK concluded that cannabis had more of a negative impact on sperm production than other lifestyle choices (like tobacco, alcohol and even tight-fitting underwear), doubling the risk in men under age 30.  

Specifically, sperm morphology – size and shape – is alleged to be affected by cannabis use, and when less than four percent of a man's sperm has normal size and shape, it is statistically harder for him to father offspring. Normal shaped sperm are able to "swim" more efficiently to a woman's egg in order to fertilize it, while irregular sperm swim less. 

In all, researchers analyzed 1,970 men who attended fertility clinics across the UK. Investigators compared the medical history and lifestyle choices of 318 men with less than four percent normal sperm versus 1,652 men with sperm of "normal quality" (above four percent). Men in the first group were almost twice as likely to have used cannabis in the three months prior to giving a sperm sample to a clinic. 

However, it should be noted that some scientists don’t consider this study’s link between pot and abnormal sperm to be a “smoking gun.”

After all, alteration of sperm normality in young men was also detected during the summer months, and researchers speculate that length of day and hormonal changes play a big role in sperm quality.