I have nothing to offer but what I've heard and anecdotal experience. People often get "mentally ill" from marijuana because it can produce mild feelings of paranoia. Add to that a country which will arrest you for it. And forcing people to buy it illegally from armed criminals, and you'll surely get people who crack under the pressure. I'm not saying that it's total bunk that marijuana causes mental problems, but I think a lot of it is attributable to its illegality.