Alcohol - its effects on me aren't all that different to what you'd expect for a "normal" person, though it doesn't take as much, probably because I don't use it regularly. 0.5 to 1 pint of Guinness, it's a mild social lubricant and it can make me a little more extravert and "confident" than usual, 2 pints and I start to feel woozy and it seems to be dulling my thinking, and it seems to numb the part of my brain that's responsible for telling me "don't do that, you'll upset somebody," though it's never got me into any serious trouble, probably because my strong self-control is very hard to override. Much more than that and I start to get balancing problems and nausea, and I just want to go home and wait for it to wear off, and I feel ill the next morning.
Caffeine - I've often tried to use strong tea in the mornings to wake myself up, as I'm often sleepy for a few hours after getting up, but I haven't noticed it's helped much. I often notice an unpleasant "poisioned" feeling (as well as the sleepiness) in the mornings, and recently when I cut out caffeine completely, that poisoned feeling is much reduced, so I suspect it could be caffeine withdrawal, but the evidence isn't conclusive yet. Unfortunately I can't seem to stop drinking huge amounts of hot tea. Decaffeinated tea seems like the answer, but the packets don't tell me whether they're using safe or dangerous methods of decaffeinating it. Luckily I can live with green tea, which is fairly low in caffeine, but like I say, I drink a lot of it, so I'm looking for safely-decaffeinated tea so I can eliminate caffeine entirely without swapping one health risk for another.
In my mis-spent youth I discovered that large numbers of Pro-Plus (caffeine) tablets helped me to stay awake when performing in a band, and I felt great at the time, but then I had a bad experience with them at a party - my heart started pounding horribly fast, and I was in a cold sweat and thought I was going to have a heart attack. After that I just stuck to tea.