When I was in my teens, the 'back-to-the-land' hippie movement was in full swing, with many moving to our rural area to 'live-off-the-land'.  We heard a lot of psuedo-scientific comments about how 'natural, organic, and healthy' marijuana was, as opposed to 'stinky beer or wine' (also made by a natural process).  There are several sites which support the premise that long-term marijuana use (like long-term heavy alcohol use) impacts the brain, and in the young impairs development.  I don't believe either pot or alcohol are necessarily worse.  I stand by my statement that we should not assume marijuana is innocent, especially with so many people finding ways to increase its strength.