I've read of pot causing mental problems due to its 'sticky' nature in the way it interacts with brain tissue. One of the biggest culprits according to that long ago reading, was the use of the drug by people whose brains were still developing, ie. youth and young adults. But, if I am also recalling correctly, that was a medical article in a Reader's Digest, so, I'm not sure where the study came from, if it was properly controlled, etc.
I recall seeing something on television about that, may have been W5. What I recall the theory being is that THC is more concentrated, more potent, in 'today's' pot as compared to what was being smoked in the 60s and 70s. They had three teens spotlighted on the program that, apparently, had a psychotic break the first time they used. Don't know how scientific it was or whether that theory has since been debunked, but that's what I recall from the program.