From Halopedia, the Halo wiki
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 as more powerful drugs.
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