Mildly:
-2 effective decrease to Dex, Int and Wis and the ability to become disabled at -1 instead of 0.
Moderately:
-4 effective decrease to Dex, Int, and Wis, the ability to become disabled at -3 instead of 0, and partial actions only. Spellcasting requires a Concentration check (DC ?) or suffer a mishap (per scrolls).
Severely:
-4 effective descrease to Dex, Int, and Wis, -2 effective decrease to Str and Cha, 1 point of Dex and Wis damage (Fort DC 11) per extra drink, the ability to become disabled at -5 instead of 0, partial actions only, and a +1 morale bonus to attack rolls. Spellcasting requires a Concentration check (DC ?+10) or suffer a mishap (per scrolls).