Stim-pack

Stimulation Packs, also known as Stim-Packs, are a type of drug used by UNSC Marines.

Background
Stim-Packs are taken orally and designed to "wake up" the user with stimulant drugs. This is primarily used on the battlefield after extended periods of combat, when fatigue starts to set in. It is unclear what drugs are contained in the stim-pack, but they appear to be stronger than caffeine (used in coffee), yet not as addictive we more powerful drugs.

Halo: The Flood
After four days of straight fighting during the Battle of Installation 04, even the Master Chief relied on these to get him through while fighting the Flood in the Library.

Halo: Combat Evolved
At the beginning of Halo: Combat Evolved, the crew member assisting the Master Chief mentions that he gave him a "double dose of the wake-up stim".

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