A Syringe is a simple piston pump consisting of a plunger that fits tightly in a tube. It is used for many medical procedures that consist of injections, and is usually used with a needle.
"Emergency medical supplement. Strong chemical compound capable of reverting cardiac arrest. Warning: misuse can be fatal."
— In-game description
In Battlefield: Bad Company 2: Vietnam, the Syringe is a gadget issued to the Medic kit. It seems to be the equivalent to the Defibrillator for the base game, and like the AED's, it can be used as a lethal melee weapon.