I've had this discussion some time ago with a couple of friends and we came up with the golden rule 'two to three drinks makes you flow easier with the music, anything more and it just screws you up'.
Alcohol makes you relax and I think therefor it's easier to play with feeling because it makes you feel more susceptible/comfortable to express yourself, it becomes more natural to let those emotions flow into your music, but the feeling's gotta be there to begin with. Music comes from the heart and alcohol makes your heart more accessible.
But more than those two or three drinks influences your senses to a point where you'll notice your abilities to lessen as does your judgment over them. It's simple math really.

I don't have any experience with other drugs (well I don't really count marijuana as drugs cause it's weak comparing, but I've never played with it), I support the opinion that music has to come naturally, but it's also a fact that drugs do enhance your perception and unlock creativity. I just don't think it's wise to rely on that, especially when playing live!