A healing chem. When injected, the chem provides immediate healing of minor wounds.
”— In-game description
The stimpak, or stimulation delivery package, is a type of hand-held medication used for healing the body from minor to moderate wounds. Superior versions are available as super stimpaks and ultra stimpaks.
These chems consists of a syringe for containing and delivering the medication, and a gauge for measuring the status of the stimpak's contents. When the medicine is injected, it provides immediate healing of the body's wounds.