The Syringe is a consumable item that can be found throughout the Lab environment in resource containers as well as on shelves or desks. They come in two different forms, a yellow one called 'IIx' and a red one called 'I2-LP'.
Yellow syringes are used for a temporary (8 second) melee boost at the cost of infection.
Using a yellow syringe allows you to kill a Big Striker with one full swing to the back of the neck. Players health will appear yellow for a brief amount of time, but the effects last a total of 8 seconds.