Relapse is a gear perk that heals you when you kill an enemy who is poisoned. It gives a poison build a way to stay alive in long fights without stopping to heal by hand.
Each kill on a poisoned target restores some health. Poisoned enemies also tend to attack each other, so you often finish them while they are distracted.
Wear Heisenberg's Watch in the hands slot. Gear perks are fixed to the piece of clothing that carries them, so you get this one by wearing that piece, not by unlocking it in a menu.
Snakebite adds damage against poisoned enemies, and Toxic Gadget Pocket gives you more poison grenades to spread the status.