Neither really, the syringes you can craft have effects such as berserking an enemy, poisoning them, bleeding etc.

In its current form its not the best thing compared to any other approach but like mentioned can throw one of the berserk ones on a suicide and enjoy the effects.

It depends on the syringe you use. You can make it so they bleed for a time, you can poison them, you can even make a bloatfly pop out when they die, but to be honest the only one worth using is the one that makes them frenzied. They seem to notice being hit by syringes so the other syringes are pretty much worthless. Especially with the terrible fire rate it has.