Adrenaline Shot

The Adrenaline injector (or the syringe) is included in Left 4 Dead 2. They use pain pills' slot and give you 25 temporary health plus they make you immune to melee fatigue and the horde's slowing melee attacks, as well as increase your movement speed a great deal for a short period of time. It also increases the speed of using health kits. The health added by the injector appears to drop slower than the health from pills. Adrenaline injector is always seen in the Crescendo event in "The Park". Injecting oneself appears to give one a tunnel vision effect. This item could become valuable at the end of a finale where Survivors are trying to get on the rescue vehicle but are constantly being slowed down by the horde. Same as the Pain Pills, Adrenaline Injectors can be given to teammates using the melee button.

Using the Adrenaline causes any sounds in the game to become mono-phonic. This means that you can't really determine where someone/something is via sound.

Trivia
Adrenaline no longer causes the user's health to go above 99hp. As seen in this video: http://www.youtube.com/watch?v=3IzkVG4-IGY&feature=related. If the player has 80 health and uses the Adrenaline, his health goes to 99.