My brother used to be a big drinker at uni, but completely quit after he went on a health kick. He started drinking again, but just at weekends, and in much lower quantities (no more 16 pint nights for him). He'd be fine until Sunday, where he would be slightly wheezy, his face would be red and blotchy like he'd been sunburnt, and he had the occasional habit of passing out (normally whilst urinating).
Turned out he'd developed an intolerance to alcohol – his body just couldn't break it down any more, and it did odd things to his blood pressure. Can't remember what his condition is called, something beginning with an A. He'd get drunk on 2 pints, although his condition has eased up a bit now, and he can go up to 4.