Mentats

Mentats are a chem in Fallout, Fallout 2, Fallout 3, Fallout: New Vegas, and Fallout Tactics.

Background
This pre-war chem was created by Med-Tek. They were designed to increase memory related functions and speed up other mental processes. These red pills were a popular party drug, widely appreciated for their effect on creativity.

Mentats were sold in both cardboard and tin packagings, the latter a bit more likely to survive the nuclear holocaust.

They are, of course, highly addictive.

Fallout, Fallout 2 and Fallout Tactics
Mentats are most useful for characters with low intelligence. Furthermore, they are one of the games' more valuable items yet are relatively easy to find. This makes them also useful as an easy source of money, particularly when first starting. In addition, the +1 to Charisma can allow you to recruit a new party member if the new CH is an even number. The party member will even stay after your CH returns to the normal score.

Fallout 3
Mentats return in Fallout 3, though they only increase Intelligence and Perception, omitting the Charisma bonus. As in previous games, using them boosts skills based on those attributes (although not to skillpoints earned on level up as Mentats do not alter base stats).

Mentats are important at the beginning of Fallout 3; particularly with disarming (or alternatively detonating) the Megaton bomb, as indirectly implied by Lucas Simms. Through the base adjustment to Perception, the Mentats can help raise the PC's Explosives skill level above *Berry Mentats grant a +5 to Intelligence
 * Orange Mentats grant a +5 to Perception
 * Grape Mentats grant a +5 to Charisma

Given that regular Mentats provide the same type and strength of bonuses as the Berry and Orange versions (and all in one package) practical-minded players may want to choose Grape Mentats when redeeming their prize vouchers. The unique +5 bonus to Charisma (and consequent +10 to Speech and Barter) may come in handy with difficult speech checks or large bartering sessions.

Behind the scenes

 * Their name is a reference to Frank Herbert's Dune, where, in the wake of humanity rejecting artificially intelligent computers and robots, Mentats are humans trained for computer-like mathematical and logical computations. Also of note is that the pills are colored red. Red-stained lips are a distinguishing characteristic of a Dune Mentat.