Drinking Contests (3.5e Variant Rule)
From D&D Wiki
Many sword-and-sorcery stories involve a drink-off between two characters. You probably know at least one player that has attempted such a thing. With this rule, when a player decides they want to out-drink the burliest dwarf in the tavern, you can make it a challenge!
Step One - Determine Alcohol Proof
The first step is to figure out the strength of the alcohol you're drinking. This is calculated by a Fortitude save DC that the contest starts at. I'll list some typical examples here and the starting DC's of each for your convenience.
Step Two - The Contest:
Once you've selected your brew, it's time to drink! The first shot requires a Fortitude save from all participants at the base DC shown above. If the save is made, the drinker suffers no ill effects. If the save is failed by less than 5, the drinker is now "tipsy", taking a -5 penalty on all attack rolls, to Armor Class, and to all skill checks. If the save is failed by 5 or more, the drinker passes out and is unconscious. If the save is failed by 10 or more, the drinker is poisoned by the alcohol; alchohol poisoning is an ingested poison that deals 1d4 Con damage (initial) and 1d6 Con damage (secondary). On each successive round, the DC for the alcohol imbibed increases by +1.