User blog comment:Miskos3/Fairy Tail Episode 238: Tartaros arc: Immorality and Sinners, Review/@comment-24039234-20150620103048/@comment-7418318-20150620120632

Or maybe Crawford is just very skilled with poisons/drugs and chose one (whether by knowledge or chance) which even Demons aren't immune to.

Considering he allied himself with Demons, it surely wouldn't hurt to have such an ace up his sleeve for contingencies.