Another issue with the study is that it doesn't compare regular marijuana users to non-users, it simply compares non-users to people who have used the drug at some point in their life. Therefore, it could be that men who naturally have more testosterone and higher sperm counts are more likely to take risks and give marijuana a try.
“Our findings were contrary to what we initially hypothesized,” said lead author Feiby Nassan. “However, they are consistent with two different interpretations, the first being that low levels of marijuana use could benefit sperm production because of its effect on the endocannabinoid system, which is known to play a role in fertility, but those benefits are lost with higher levels of marijuana consumption.
“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 behaviors, including smoking marijuana.”
The team concludes in their paper that moderate marijuana use may improve testicular function, but that this likely reverses at high doses resulting in “adverse effects”. So no, smoking pot all day won’t give you super sperm, but small amounts may have some benefit. We need more research to find out.