My take, after years of fiddling and drinking: I dare not touch a drop before a gig begins. A couple of good swallows is excess. It doesn't hurt the singing, but makes the fingertips dull and slow. Once I've begun to sweat, it's different; hands are working, and in the break I grab a glass for nourishment during the second set: it seems to strengthen the heart. That's usually the limit, until quitting time, when everyone goes home and I have a huge thirst. I remember Bukka White sitting in our house years ago with his private fifth of Jack Daniels. He seemed to just get better/wilder/better until it was empty. He must have had quite a habit.

One other subject was mentioned, Beta Blockers. I took them for several years because of chronic headaches (which were caused, we eventually learned, by consuming sodium nitrite preservative in certain meats). I had the distinct impression that during that period I was less bright & less creative: they took the edge off. I know the literature says it is not an effect, but personally I would never take them if I could avoid them.