"These inbuilt chemical delivery systems dose marines with a powerful mix of synthetic adrenaline, endorphins, and a psychotropic aggression enhancer. Marines on stims benefit from greatly increased speed and reflexes, but are subject to long-term side effects including and not limited to insomnia, weight loss, mania/hypomania, seizures, paranoiac hallucinations, internal hemorrhaging, and cerebral deterioration. Nonetheless, both commanders and the marines themselves stand by the use of stims as essential to their continued survival and effectiveness on the battlefield."
The stimpack is an in-field chemical delivery system filled with a variety of stimulants. These devices are featured as part of the CMC-200, CMC-400, and later-model CMC-660 combat suits. They are widely used by terran infantry. 
Stimpacks were commonly used by terran infantry to help improve combat performance, and were in use as early as the Guild Wars. However, these devices commonly had drastic side effects, including cellular degradation, addiction, weight loss, and hemorrhaging.Medics could help mitigate the side effects.
Although the stimpack is built into these armors, smaller, portable stimpacks exist. These stimpacks can be applied directly to the neck and are compatible with CMC-200 combat suits. A resupply unit is located near the shoulder of the armor. The chemical delivery system is known to be rather loud, even in the middle of combat.
Historically, administering stimpacks to high PI subjects such as ghosts was considered extremely dangerous. However, some ghosts were known to requisition stimpacks for battlefield operations without command authorization. By the time of the Defenders of Man Insurgency, improved stim formulation alleviated most of those concerns.
Stimpacks will not activate if the unit has 10 or less hit points left. Activating stimpacks again before the previous use has worn out resets the timer and subtracts additional hit points; the unit's cool down does not decrease further.
Stimpacks in StarCraft II can be used by marines and marauders in multiplayer, as well as by firebats in Co-op Missions. Bunkered marines can use their stimpacks while fighting in the bunker. Unlike marines, marauders take 20 health damage per use of the stimpack instead of 10.
In the StarCraft II: Wings of Liberty single player campaign, Tychus Findlay was also intended to have been given a variant of the Stimpack ability named "Tychus' Secret Stash". It is described instead as a mix of rocket fuel, methamphetamine and various chemicals. Only the button remains in the map editor.