> Caffeine - never been an issue for me - I feel like it has no effect whatsoever. But the people who say "don't speak to me before I've had some coffee" certainly sound like I did when I was using cocaine.

I don’t have notable experience with addictive-type substances other than caffeine, but I can chime in there - I went cold-turkey from drinking 4-5 cups of coffee per day, and I was pretty surprised by how awful I felt the first day. Two days later I felt perfectly fine without caffeine.

Now I drink one cup per day, only before noon, often decaf. If I skip a day, I don’t notice any effect.