Using Buffout carries a 25% chance to become addicted. The addiction lasts for a week, beginning 48 hours after consumption. When addicted, you suffer a -2 Strength and Endurance penalty and a -3 Agility penalty.
In Fallout this addiction is refereed to as "Buffout Craving" while in Fallout 2 it is refereed to as "Buffout Reliance".
Fallout 3, Fallout: New Vegas
Using Buffout carries a 10% chance to become addicted. The penalties of being addicted have been significantly lowered, as being addicted only causes a -1 penalty to Strength and Endurance.