From Battlestar Wiki, the free, open content Battlestar Galactica encyclopedia and episode guide
Stims, short for "stimulants", are a type of drug used to temporarily increase alertness and attentiveness. Nothing yet is known of their chemical composition, or if the term refers to one substance or a class of several.
Galactica's pilots are forced to use them in an attempt to stay alert after four continuous days on Condition One and Two alert status after their initial escape from Ragnar (TRS: "33"). During this time, when Lee Adama, Kara Thrace, Sharon Valerii and Alex Quartararo are flying CAP, Crashdown describes the feeling of stims as "ants crawling behind his eyeballs."
Overuse or overdosing on stims can cause irrational, violent, and psychotic behavior. Viper pilot Louanne Katraine severely damages her Viper and injures herself when her stim use reaches a point where she can barely fly or understand instructions (TRS: "Final Cut"). Later, when the use of stims is suggested during a particular grueling mission, Katraine argues strongly against their use, stating it would tax the starving pilots' system too greatly. Stim use for the mission is then left up to individual discretion (TRS: "The Passage").
Stims are a likely component of a Colonial medkit.