That's like thinking that Plan Nine From Outer Space is based on real observations of space aliens.
As I recall, the movie "Reefer Madness" showed a normal young man who became delusional and paranoid as a result of marijuana use.
Current research is showing that marijuana use causes brain damage that may very well be permanent. Users become paranoid (and this effect was well-known among users back in the 1970s). People's short-term memory is wiped out. Although these effects are quite noticeable during the period of intoxication, there is evidence that these effects--especially the loss of memory and lack of initiative--linger long after the "high" has dissipated. Marijuana triggers psychotic disorder (whether it causes it has not been determined) in people whose brains are still developing (brain development is complete around age 25). It physically changes the structure of the brain.
As I said before, it looks more and more like Reefer Madness may have been a bit exaggerated, but it wasn't that far off from the truth.