Well, I know kinda lot of marihuana users, some heavy one, but those who do it regularly are just kinda {bleep}ed up, I mean, kinda like alcoholics, but I've never seen nor heard about a real case of schizophrenia caused by heavy use of marihuana. So, how much of this research has been proven, I wonder.
If we take a look at the past, humans have always lived with drugs, all kinds of drugs (no not XTC or LSD, but maybe natural drugs wich equals these).
So it has to be that the human body/brain has adapted to many of these drugs.