Apparently, and surprisingly, this is a hotly debated issue?
Well, that being the case, I'll just make my comment and move on:
There's nothing wrong with having a few drinks and getting a LITTLE tipsy (Even the Bible says "Wine doth gladden the heart," and therefore being a little tipsy can't even be a venial sin).
Drinking would become venial when done intemperately, or mortal when done to the point of surrendering the rational faculty.
But as regards marijuana -and feel free to correct me if I'm wrong- for the last 30 years, the strength of it is such that, unlike alcohol, your faculty is severely modified and distorted after only a couple puffs. Even for daily user addicts who are accustomed to its effects, its not that it no longer has a severe affect upon them, or that they have become tolerant to it, but rather that they have become acccustomed to long-term distortion of their rational faculty.
I believe they are AT LEAST in the same (or worse) state of mind as a drunken man, and nobody disputes that total drunkenness is mortal.
As for the real or alleged health benefits of marijuana, I find that completely irrelevant to the question of its morality (and besides, it is available in non-euphoric form anyway).
Crack, speed, and meth are probably good for weight loss, since they speed the metabolism and suppresses the appetite, but that doesn't make them moral.
Flame away!
PS: I think modern man is afflicted by misery, and this accounts for even some Catholics defending marijuana use: They want something to kill the pain, and take a break from reality. But if true, doesn't that prove the point?