If you are a woman of childbearing age and you try the antibiotics excuse then everyone will automatically assume you are pregnant . In fact, if a man did it I'd almost suspect he was pregnant...
I really think that the "I have the world's worst hangover" excuse is the best one, but you'll have to build up to it -- start talking now about how you are going round to the house of one of your friends who isn't a work friend for a party the night before the work do, how she's a real party animal and you're surprised she's having a party on a work night, frankly, because last time everyone ended up with 24-hour hangovers, blah di blah di blah. Then pick up a big bottle of wine on your way into work (or better yet pop out to get one in your lunch hour) on the day of the "party" to have as your "bring a bottle", and make sure everyone sees it. Then stagger in (preferably a bit late) the morning after the "party" (i.e. day of the work do) and tell everyone how you are never ever drinking again and you wish you were dead. Have suitable stories of your feats of binge-drinkiness from the night before but bemoan that you must be getting old because you can't take it any more.
Ideally then when you are at the work do if you are buying the round get a non-alcoholic cocktail that looks suitably alcoholic. Then if someone else is getting a round say that you're pacing yourself after last night and you'll just have a coke this time. Once or twice let someone buy you an alcoholic drink, take the teensiest tiniest sip (just wetting your lips, not even really enough to swallow) and then lose it (after a while, wander off to talk to someone, put your drink down snd leave it).