Stim-pack
From Halopedia, the Halo wiki
Stimulation Packs, alternatively known as Stim-Packs or Wake-up stims, are a type of drug used by UNSC soldiers. They are taken oraly, 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 caffene (used in coffee) yet not as addictive as other, more powerful drugs.
After four days of streight fighting on Installation 04, Even the Master Chief relied on these to get him through.
