From Halopedia, the Halo wiki
Stimulant packs, also known as stim-packs or combat stimulants, are a type of drug used by humans.
Stim-packs see use in both the military and civilian fields. They are commonly employed as standard-issue equipment in the UNSC Armed Forces, especially by special forces troops who often had to fight for multiple days at a time. The Militia of Mesra, however, had an extreme aversion to the use of stims, and the suggestion they were using them was something considered deeply offensive.
In the civilian world, stim-pack usage is considered a form of cheating. In one instance, the winners of the 2549 Tantalus 10,000 race had their victory disqualified after it being revealed they had used stimulants to achieve victory, with the victory instead going to runner-up Bella Disztl.
Stimulants are designed to wake-up the user. They counter effects of sleep deprivation by temporarily increasing the signal-carrying capacity of the brain's synapses. A common sign of stim-pack overload is for a soldier to be focus-locked on a specific and pointless goal.
Stimulants can be used to quickly wake a subject from cryogenic stasis. In one case, John-117 was given a "double dose of the wake-up stim" while awakening aboard UNSC Pillar of Autumn to quickly prepare for combat. After four days of straight fighting during the Battle of Installation 04, even the Master Chief relied on stims to get him through while fighting the Flood in the Library.
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