hmmm...
weed-badspot sends you a kilo to your place haha, though it would be nice, no seriusly, should be something like clumsy movements and walking slower or faster (randomly) and if you keep doing it inverted controls or something like that 'til you pass out
coke-mmm.. your character moves faster and is stronger and if you keep going the effects multiply 'til starts shaking and loses health and eventually overdose.
acid- the world colors change and everything starts to look distorted, something like players turned into monsters and monsters into players or nice things like pink rabbits haha
mushrooms-pretty much like the acid but when the effect wears off you're still on the same place you were when you took it and the gameplay was just locally
crack-more strenght and erratic moves and the character cant do nothing but smoke til he/she has no money on
lol that would be nice, specially if you smoke a joint and play, too bad it isn't gonna be on the game