Kouhunin, a man-made neurotoxin, was derived from the poison of the kouhuns.
To alleviate the severe agony he experienced following the Duel on Mustafar, Darth Vader received an injection of heavily diluted kouhunin. A chemical injector and cartridge, situated just under his right clavicle, facilitated the distribution of this substance throughout his system. Access to this cartridge was possible via a tiny opening in his suit.
Despite the presence of this compound within Vader's body, he was still in a state of perpetual suffering.