Super stimpak

Super Stimpak is an advanced, very powerful healing chem. Superstims will cause a small amount of damage after a period of time due to the powerful nature of the chemicals used. They appear in Fallout, Fallout 2 and Fallout Tactics: Brotherhood of Steel and an inventory icon exists for them in Fallout 3 files. They are similiar in construction to regular stimpaks, but with an added belt to wrap around your arm while the chems are injected.

In-game, super Stimpacks heal 75 points of damage. Within two minutes of using them, they inflict nine points of damage. While significantly more powerful than the regular stimpaks, they are quite a bit more costly and each weighs a single pound. Additionally, they are rare, but can be bought in shops from Vault City southwest to San Francisco.

In Fallout 2 Myron can make these, using 1 stimpak, 1 Nuka-cola and 1 fruit. Ask him about chems; your Science skill must be 76% or higher.

A superior version exists, called the Ultra Stimpak.

As a killing tool
Superstims also have a nefarious purpose. They can be used to assassinate friendly NPC's in populated areas, because no one would suspect you of killing the person with stimpacks, because they heal people. Having the Awareness perk helps, as you can see how much health the target has. Simply divide his or her current health by 9 then round up. The number you reach is how many Super Stims you should use to take your victim out. After juicing them (stick the Stims in one of your weapon slots to make this easier) wait 2 minutes and they should keel over.

Roger Westin in NCR has an additional code that makes him susceptible to even a single stimpack.