Lincoln will start with two health bars which can be further upgraded through Vito's favors. Each bar regenerates as long as the damage inflicted to him doesn't exceed one bar. If all bars are depleted, Lincoln will have a "last shot" status where the screen turns white and any damage may instantly kill him. Always wear a Tac-vest especially in the later part of the game where the enemies have access to more powerful guns.
Adrenaline Shots
These consumable items are found in Medicine Cabinets scattered in various locations. You can only carry a set number of them but this can be further upgraded as you increase your underbosses' earn and gain access to more of their favors. Adrenaline shots can instantly refill up to four health bars when you're not taking damage.
Tac-Vest
This body armor provides additional protection against bullets, explosions or even fall damage. The armor wears down as you take damage, indicated by the thin blue gauge above the health bar. These are found either from Weapon Lockers or on the floor.
Hideouts usually contain both medicine cabinets and weapon lockers. Normal establishments usually carry them as well but you may have to trespass inside a store or restaurant's back room where the weapon locker or medicine cabinet is located. If you have wiretapped an area, these will appear in your minimap alongside the fuses and collectibles.