Chem Resistant (perk)/Fallout 3

Chem Resistant is a perk in Fallout 3, Fallout: New Vegas, and Fallout 4.

Fallout 3 and Fallout: New Vegas
Having the Chem Resistant perk means you're 50% less likely to develop an addiction to any chem, like psycho and jet or alcoholic drinks like whiskey and vodka.

Fallout 4
The first rank of this perk reduces the chances of being addicted to anything by 50%. The second rank negates all chances of become addicted.