While in combat, enemy NPCs have a chance to use a tonic and heal themselves, like the player. Healing chance and amount are different depending on ped type (cowpoke, law, pinkerton), the tonic also gives them increased accuracy and fire rate (can be toggled off).
You can also choose whether they use it while out of cover or not, and many other options. Fully configurable via the ini file.
To install drag and drop both the .asi and the .ini files to your game directory
Credits:
somncoitus