Studies of people going insane from smoking weed might be a bit skewed don't you think? I mean its not as if they actually know how much is sold and how many people smoke it.... its illegal! Most of the studies don't claim that it sends everyone insane either, but state it'll cause worse effects if the person was prone to that anyway.
The BBC recently stated that a study found "cannabis users 40% more likely to have a psychotic illness"... 40% more like than what exactly?
The media don't really portray this but put some evil backdrop up with some hoodie youth stabbing a grannie while smoking a spliff (its always a really badly rolled one at that), with the words CANNABIS, MENTAL PROBLEMS, GIVES, CAUSES, INSANE, DEATH.
How many nutters do you see drinking themeselves to an early death each day? Don't see many evil backdrops of a tramp mugging some middleclass woman getting into their Mercedes just so they can get another bottle of White Lightning, do you now?