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 Tactics
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.
In Fallout this addiction is refereed to as "Mentats Craving" while in Fallout 2 it is refereed to as "Mentats Reliance".
Fallout 3 and Fallout: New Vegas
Using Mentats carries a 5% chance of becoming addicted. When addicted, you suffer from a -1 penalty to Intelligence and Perception.
Known Mentat addicts