I also think that everyone has a Decency score, where
Net Shittyness (NS) = IS + Decency
Some drugs remove the Decency filter such that NS = IS. If a person knows they have a high IS, and that, say, alcohol temporarily lets that show through, and they care about their perceived NS, then it behooves them to stay away from it.
Δ Shittyness = IS x drug amount x duration
Sometimes people get off illegal drugs and become decent again, but