You may not quaff potions if you:
- Are a mummy
- Are berserk
- Are transformed into a monster which cannot quaff: Necromutation, Tree Form, Wisp Form and Bat Form
- Are retching
- Have recently cast Ozocubu's Refrigeration
Prior to 0.15, Ozocubu's Refrigeration had no impact on your ability to quaff potions (but did occasionally destroy them in your inventory).