When using chems or alcohol, the effects of these substances disappear after some time. If the user 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 chems), or the withdrawal persists.
During this period of withdrawal, some of the stats are lowered, often the ones that were increased under the influence of the chem that gave the addiction. Some perks and traits, like Psychotic or Chem Resistant, have an effect on these withdrawals.