When using chems or alcohol, the effects of these substances disappear after some time. If the player becomes addicted to a substance, withdrawal will occur when the drug is no longer in their system. In both Fallout 3 and Fallout: New Vegas, addictions must be treated by a doctor (or, temporarily, with more drugs), or the player will continue to go through withdrawals.
During this period of withdrawal, some of the player's stats are lowered, often the ones that were increased under the influence of the chem to which the player is addicted. Some perks and traits, like Psychotic or Chem Resistant, have an effect on player withdrawals.