@rdr Yeah there's a study that was about half regular marijuana users and half healthy adults (thus non-smokers) and they found that in the regular marijuana signals took about 10ms longer to travel over the optic nerve from the retina to the visual cortex. 

It seemed to be permanent as months after stopping marijuana use the higher latency was still present. They concluded marijuana use might damage the optic nerve. 

However, they made no real effort to control anything. 

When did these people start using marijuana? Chances are when they were teenagers and we know marijuana use in teen years has lasting changes. It's debatable if the changes are actually all that harmful tho. 

Second it could be a comorbidity because people born with higher latency optic nerves may be more likely to use marijuana. 

Third it could have nothing to do with marijuana and be a side effect of inhaling combustion byproducts like carbon monoxide. 

Fourth where are they getting their weed? If illegally then all bets are off because it could be that they once all got weed contaminated with fentanyl or some kind of solvent or something and all it takes is one time use of that contaminant to cause permanent damage of the optic nerve.

There are way too many unknowns to draw any meaningful conclusion other than there seems to be a comorbidity for an unknown reason.