It has been known for a while that certain drugs (also LSD) can induce psychosis' in certain people. The problem is that you cannot know if you are one of these people until you tried for a while. So it's a bit like playing Russian roulette. If you don't develop a psychosis when doing LSD, marihuana or shrooms, chances are that you will be fine. But if you do, you have a problem, possibly destroying your whole life.