It wouldn't make any noticeable difference, as soldiers (and insurgents)take all practicable measures to avoid being shot already. Maybe expect even more IEDs, RPG and mortar attacks but those are already vastly more common than small arms attacks.
Note however that poison bullets would have rather limited effect: the vast majority of casualties are caused by others means -- specifically artillery and mortars or various types, along with a variety of rockets (from should-launched upwards) and guided missiles (again, from shoulder-launched upwards) as well as the ubiquitous grenade launchers. The general idea it to avoid your footsoldiers getting close enough to the opposition to get shot.
Legality is perhaps the biggest issue here, along with toxic remains of war. How do you clean up the battlefield with all these deadly poisonous munitions afterwards? How long do they remain poisonous for?
What happens when a bullet hits a solid object (eg concrete wall, do you get lethal toxic powder remnants?)