Addiction can happen from using too many drugs. An addict suffers withdrawal symptoms that can cause ill effects when a drug wears off. Many substances in the Fallout games are addictive. Each chem has a chance of addiction. One of the most addictive drugs is Jet.
For Fallout 1 & 2, withdrawal effects occur after becoming addicted and going for some amount of time without using that chem. For non-jet addictions, addiction lasts 7 days from the onset of the withdrawal effects (nine days of non-use), while jet addiction lasts forever without the Jet antidote.
Certain NPCs have addictions, not just the player character.
In addition to addiction chance, certain items have an additional, increasing chance of addiction with each subsequent use. Internally, this effect is referred to as the Usage Monitor Effect, or UMON. Each use of an item adds its UMON value to the total UMON value before the addiction check is performed. This value remains added to the UMON total until its specified time limit elapses, at which point it is removed from the current running total. An accumulated value of 75 appears to equate to an additional 1% chance (added on top of any existing addiction chance), while a value of 175 appears to be equivalent to a 100% chance. The specifics of the algorithm remain unclear.
New drugs and addiction-causing substances were added to Fallout: New Vegas, many with their own new forms of addiction. Due to the engine used in Fallout: New Vegas, addiction behaves the same as it did in Fallout 3. It is permanent unless seen to by a doctor. Fixer will provide temporary relief from the symptoms of withdrawal. When Old World Blues is installed, the Sink Auto-Doc cures addictions for free as part of the basic physical exam. For information about UMON, risk and so on, see the appropriate Fallout 3 section.
pcps3 Completing Dead Money with an addiction, it is possible for it to become permanent.
pcxbox360Fixer can permanently remove addictions in an unpatched game.
pc Ant nectar addiction used to be incurable due to a bug, but this was fixed in Fallout: New Vegas patch 22.214.171.124x. The console command Player.RemoveSpell 0006dc6e will immediately remove the addiction on the PC version.