You used the chem Mentats once too often and now your body is craving for more... and more... and more... well done! You got yourself addicted.
Mentats addiction is a form of addiction which can occur in Fallout, Fallout 2, Fallout 3, Fallout: New Vegas and Fallout Tactics.
Fallout, Fallout 2, Fallout TacticsEdit
Using Mentats carries a 15% chance of becoming addicted. The addiction lasts for 1 week. When addicted, you suffer from a -2 penalty to Agility and a -3 penalty to Intelligence.
Using Mentats carries a 5% chance of becoming addicted. When addicted, you suffer from a -1 penalty to Intelligence and Perception.
Fallout: New VegasEdit
In addition to the Mentats as above in Fallout 3, the Player can craft Party Time Mentats that in exchange for a Charisma bonus of 5 have an addiction risk of 15%, at otherwise the same effects.