With people who knew me - I would use the excuse that I was on some heavy duty antibiotics (could be bladder infection - usually people don't question though). I would mention it almost immediately, before the subject of drinking was even brought up. Then I could easily add in that I'm not allowed to drink on these meds. Obviously it's not a permanent excuse, but it's a good temporary one to use if you're not ready to be honest and bring up the whole "alcoholic thing" - better safe than sorry.