[Gef]

Shaken, not Stirred: You can quaff mixed drinks - of potions - without ill effect, albeit no guarantee that you'll gain any benefit. This is a variant of the Immunity perk in Power-Ups 2.

Off the top of my head, I'd nominate a flat -5 penalty to HT to get the benefit, and a roll higher than unmodified HT has bad consequences:
1 - replaces original potion
2 - cancels original potion
3 - induces nausea
4 - cancels original AND induces nausea
5 - induces nausea and inflicts some damage (say 1d-4, 1d cycles every 10min)
6 - As 5, plus cancels original potion

-GEF