For example, you might have some herb combination that "restores HP", but whenever you use it, you strangely lose HP that more than cancels what it gave you.

What if instead of being useless (by having an additional cancelling effect), magical potions etc. had no effect at all? If HP isn't explicitly stated, you can make the player feel like he's regaining health (e.g. by some visual cues), but in reality he'd die just as often.