Adrenaline

The Adrenaline powerup in Doom 3 resembles a hypodermic needle. Once taken, it is immediately used, and the sound of an accelerated heartbeat is heard. For the next 60 seconds, the player will always have a full gague of stamina, allowing him to run continuously.

A similar effect occurs whenever the player is in Hell (this includes the Hell level and the final fight with the Cyberdemon); this is noted as an effect caused by Hell itself and is visually indicated by flames shooting across the stamina gague.