I make it a policy to never actually confess to anything on line, but let's just say that the various groups I game with all have one or more stoners in them, and at least one person grows weed, so the smoke flows freely during those groups for those who wish to indulge.
We also do a fair amount of drinking during gaming, at least some of us, some of the time.
My way of dealing with the giggling fool issue that Moruss mentioned is to never give a player a break for being drunk or stoned. Never ever. (And that would include my own mistakes, hypothetically speaking, if I was the drunk and/or stoned one).
As far as in-game drugs... oh my, yes. I have a whole slew of different fantasy drugs available for the daring or foolish to try, from dzur (literal rocks that dwarves smoke) to hempflower (weed) and blackroot grass (a super narcotic, super addictive, mind crushing, soul wrecking grass you chew if you want to lose your character's health and sanity). Krahk, goof balls, nose candy... if it sounds like it could be some sort of drug, it probably exists in my game somewhere.
A balor at a party in Sigil once passed around a bong that was loaded with some sort of foul smoking herb that turned the inhaler Chaotic Evil for an hour.