I don't think that it's that alcoholics don't get sick, it's just that they get used to those feelings so they can cope better. People are capable of adapting to a lot. I've heard it creates this cycle where you wake up feeling hungover and start drinking because you both want to distract yourself from feeling terrible and because "hair of the dog" is a concept people believe in. I've definitely seen alcoholic friends drink enough to puke. It just doesn't seem to hit them as hard as it would me. A lot of them start reporting black out periods more than just throwing up and passing out, which is really scary to me. I've never blacked out but I don't want to.