>People change behaviour almost immediately. I've noticed that with a drink myself; I'm feeling the drink before I possibly could have absorbed it.

I have noticed a similar thing in myself. The second I taste alcohol, it is like a switch was flipped, and I am now "drinking mode". Presumably some strong state-dependent learning. On the other hand, if I am physically dehydrated and drink a glass of water, I immediately feel better even though it must surely take several minutes for the water to start being absorbed.