bestofwikipedia:

Mithridatism is the practice of protecting oneself against a poison by gradually self-administering non-lethal amounts. The word derives from Mithridates VI, the King of Pontus, who so feared being poisoned that he regularly ingested small doses, aiming to develop immunity. Having been defeated by Pompey, legend has it that Mithridates tried to commit suicide using poison but failed because of his immunity and so had to resort to having a mercenary run him through with his sword.