One widely circulated 2014 study involving almost 2,000 British men - the world's largest study to explore how common lifestyle factors influence sperm morphology (the size and shape of sperm) - found that males under 30 with less than four-per-cent normal sperm were almost twice as likely to have used cannabis in the previous three months.
"An equally important limitation is the fact that most of the data were collected while cannabis was illegal in MA, so it is hard to know to what extent men may have under-reported use of cannabis because of social stigma or potential consequences related to insurance coverage for infertility services", he said.
Men who had smoked marijuana had more concentrated sperm than men who had never smoked marijuana, the researchers found. Sperm concentration was almost 63 million per milliliter for men who had smoked versus only about 45 million per milliliter for those had never smoked, the team reports today in the journal Human Reproduction.
They believe improved fertility in moderate users may be explained by a boost to the endocannabinoid system, known. Among men who had ever smoked cannabis, those who smoked it more often had testosterone levels an average of eight nanograms per decilitre (ng/dL) higher than those who used it less often.
While cocaine use was associated with a higher estimated proportion - 12 per cent having sperm concentration below World Health Organization reference values.
"An equally plausible interpretation is that our findings could reflect the fact that men with higher testosterone levels are more likely to engage in risk-seeking behaviours, including smoking marijuana".
He said: "These unexpected findings from our study highlight that we know too little about the reproductive health effects of cannabis".
The men involved in this study reported their own marijuana use, answering questions about whether they currently smoked weed, how much they smoked weed now, or if they had ever done so in the past.
Numerous older studies had focused on animal models or had examined men with histories of drug abuse. By comparison, 12 percent of men who never tried marijuana had below-normal sperm levels, the findings showed. In addition, a study from previous year found that marijuana's active ingredient, THC, can change the structure and development of a sperm's DNA.
There's also a potential link between pot use and testosterone, said senior researcher Dr. Jorge Chavarro.
The jury may still be out on whether marijuana use has an effect on a couple's ability to get pregnant, but it's worth keeping all of this research in mind if you're hoping to start a family anytime soon.
But the authors also say there are a few legitimate reasons why marijuana use really might be tied to higher sperm quality.
It is estimated that 16.5% of adults in the USA use marijuana, and support for legal recreational use of marijuana has increased dramatically in recent years.
'Moreover, nor does it give support to any apparent fertility benefits of smoking marijuana.
Just over 50 per cent of the men reported using cannabis; half of these men reported using it in the past, while 11 per cent said they were current users.
The researchers also noted that their study was conducted among men who visited a fertility clinic, and so the results may not necessarily apply to the general population.
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