At this point Dean is still taking all of his meds, right? (Because he says tomorrow he and Sam will start cutting down on them.) He's been thinking really clearly compared to how he was reacting to everything at the beginning when his mind was foggy on the same pills. I'm thinking either he adjusted to the meds pretty quickly (in which case they're actually heling him and he should keep taking them) or he's already started cutting down on his doses.