Doctors are just as often behind in the research as the rest of us. The general concensus among the FDA, DEA, and so forth is that it makes you go crazy because "studies show" as such, even if those studies are later debunked. Most reasoning from people still convinced of the reefer madness is "He smokes weed? He went crazy? Weed therefor made him go crazy!"

I'll get you some studies once I can get my computer working properly. I'm just having some videocard troubles that's keeping me from opening webpages 90% of the time.