“Having the Chem Resistant perk means you're 50% less likely to develop an addiction to chems, like Psycho or Jet.”— Fallout: New Vegas in game 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.
Edit
