Kouhunin was a synthetic neurotoxin based on the venom of kouhuns.
Darth Vader was injected with a highly diluted amount of kouhunin in order to suppress the intense pain that he underwent after the Duel on Mustafar. It was distributed to his body via a chemical injector and cartridge located just below his right collarbone. This cartridge could be accessed through a small gasket in the bodysuit.
Even as Vader's body possessed this substance, he remained in constant torment.