My opinion if this:
If it interferes with your playing, then you're going too far.

Drunk, stoned stupid, or drooling on yourself because you injected Heroin into your arm; you're equally useless on stage. I also think that taking something to get over nerves is a bad thing - you're creating a dependance you don't need and ultemately crippling yourself in the long run. That isn't to say I won't have a drink or maybe two before I go on and nurse a drink per set (because I do), but it's not because of nerves.

I myself try and stay away from anything I don't think I could quit at any time I wanted.