In my opinion, there is no problem with many psychedelics in and of themselves unless a person acts with
heedlessness because of them. They don't help much with meditation either. (Then again there are some Vajrayana practitioners in the Ngak'phang lineage who would suggest otherwise.) Marijuana, along with other psychedelics, can recreate some experiences gained hrough meditation and absorption in the jhanas, such as loss of ego and self, and insight into emptiness, but this is never lasting. At most you get 8 hours out of magic mushrooms. The biggest problem is when people equate psychedelic experiences gained through the use of substances like marijuana with enlightened experiences. There simply is no real connection between, say, LSD and actual kensho. However I see no problem with
occasionally using marijuana for enjoyment. It doesn't, in my opinion, go against the fifth precept, especially if done in moderation, with mindfulness and right intent.
In terms of medicinal use, it is much healthier to vaporize Cannabis or to eat it than to smoke it. Smoking too much Cannabis can cause emphesema.
"Nothing can be known, not even this."
-- Arcesilaus (but I'm not sure)
(cutting through bullshit, and sometimes failing... that's ok, though)
http://thegrandtangent.com