The Adrenaline powerup in Doom 3 resembles a large hypodermic needle. Once taken, it is immediately used, the player gives out a sigh, and the sound of an accelerated heartbeat is heard. For the next 60 seconds, the player will have infinite stamina, allowing them to run continuously.
Whenever the player is in Hell (this includes the Hell level and the final fight with the Cyberdemon), they are able to sprint indefinitely without exhausting. This is an effect caused by Hell itself (dubbed "the Hercules complex" by UAC scientists) and is visually indicated by flames shooting across the stamina gauge.
An Adrenaline pickup is not indicated in the HUD, so many players don't realize what the pickup is or what it does.