Chem Resistant

Chem Resistant (Clean Living in Van Buren) is a Fallout, Fallout 2, Fallout Tactics and Van Buren Trait, as well as a Fallout 3 and a Fallout: New Vegas Perk.

Fallout, Fallout 2, Fallout Tactics and Van Buren
Chemicals only affect you half as long as normal, but your chance to be addicted is also only 50% of normal.

Fallout 3, Fallout: New Vegas
Having the Chem Resistant perk means you’re 50% less likely to develop an addiction to chems, like Psycho or Jet.