Mentats

Mentats is a Fallout, Fallout 2, Fallout 3 and Fallout Tactics drug.

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).

In The National Archives, specially-flavored Mentats were given to anyone who won a trivia contest regarding the Declaration of Independence. This contest still exists in electronic form for the player to attempt. These bonus flavors of Mentats provide a temporary +5 boost to a single S.P.E.C.I.A.L. attribute:
 * Berry Mentats grant a +5 to Intelligence
 * Orange Mentats grant a +5 to Perception
 * Grape Mentats grant a +5 to Charisma

Behind the scenes

 * Their name is a reference to Frank Herbert's Dune, where, in the wake of humanity rejecting computers mimicking human minds, Mentats are humans trained to become living computers, also of note is that the pills are colored red- Red Lips are the characteristic of a Dune Mentat.