The advanced version of the stimpak boasts stronger healing drugs, delivered both via the syringe and a tributary ampule, both connected to a leather strap allowing to securely affix the contraption to the injection area. The analogue gauge displaying information is on top of the ampule. The powerful nature of the drugs used means that the delayed shock deals a minor amount of damage to the user after some time. It is a fairly rare item in the wasteland, usually only manufactured by high tech settlements like Vault City.
In game terms, the super stimpak will immediately heal 75 hit points, but cause a loss of 9 hit points after two minutes.
Use for assassination
The damage mechanic also applies to non-player characters. As such, it can be used to kill any non-player character if you have enough. Simply use them on the victim one after one and watch them keel over after two minutes. Awareness greatly helps determine the number of super stims you have to use (divide the current HP by nine, rounding up).
↑Fallout Official Survival Guide p.124: "Super Stimpak: Same as the basic stimpak, but with more and stronger drugs. It boosts your character's Hit Points immediately. Too much is bad for you, however. Each injection causes a Hit Point loss after a short period of time, reflecting damage to the character's tissues and to his or her system."