Like the other Star Cards and weapons in the DLC, the Adrenaline Stim can only be unlocked through a Hutt Contract. The name of the Hutt Contract to unlock the regular Adrenaline Stim is "Combat Rush". The name of the Hutt Contract to unlock the upgraded Adrenaline Stim is "Combat Rush II".
The player should mostly use this if hurt, so they will gain a health bonus in order to continue fighting and stay alive. One example could be if the player was injured a lot, followed by an enemy entering the fray. This also would be helpful if the player is in a massive firefight, and is caught in the middle. One should be careful with using it though, as it will have a short delay before it will continue to heal the player if they are damaged again. It is possible to survive Vader's Force Choke when Adrenaline Stim is active.