“All the rush without the hassle! Can't get enough of Jet, Psycho, or Buffout (or any other chem, including the ones you've made yourself at a chemistry station)? Then lessen (and finally remove) the downside of chem addiction completely. This is essential if you plan on enhancing you routines with chems.”— Fallout 4 Vault Dweller's Survival Guide description
Chem Resistant is added to the game with the Fallout: New California expansion. Whenever a player character is about to become addicted by taking a drug, they can instead chose to take 3 damage instead. They can also chose to discard the perk card in order to lose the Addicted trait.