Mithridates was the son of Mithridates V of Pontus, called Euergetes. Mithridates spent much of his early career as a fugitive. To clear his path to the throne of the kingdom of Pontus, he killed off many of his brothers, but not his sister, Laodice, whom he married. He was ambitious, and sought to invade a number of neighbours, including Bithynia, which brought him into conflict with the expanding Roman Republic during its later years.
A massacre of Roman citizens in western Anatolia in 88 BC brought matters to a head. During the First Mithridatic War fought between 88 BC and 84 BC, Lucius Cornelius Sulla forced Mithridates out of Greece proper, but then had to return to Italy to answer the threat posed by Marius, and thus Mithridates was defeated but not beaten. A peace was made between Rome and Pontus, but this proved a mere temporary setback. Mithridates recouped his forces, and when Rome attempted to annex Bithynia, Mithridates attacked with an even larger army, leading to the Second Mithridatic War. from 83 BC to 81 BC. First Lucullus, and then Pompey the Great were sent against Mithridates, who was at last defeated by Pompey in the Third Mithridatic War of 75 BC to 65 BC. After his final defeat in 65 BC, Mithridates fled, and attempted to raise yet another army to take on the Romans, but failed to do so.
Two curious legends are told of Mithridates VI of Pontus. First, he was supposed to have had a prodigious memory. He could speak twenty-five languages, and to be able to address each soldier in his large armies by name, and in his own tongue. This legend is alluded to in the short story Funes the Memorious by Jorge Luis Borges [1].
From this legend, several books that published samples of many languages were entitled Mithridates.
The second legend is that Mithridates sought to harden himself against poisoning by taking increasing sub-lethal doses of the poisons he knew of until he was able to tolerate lethal doses. A. E. Housman alludes to this practice, also known as mithridatism, in the poem "Terence, this is stupid stuff" in A Shropshire Lad [1]. On account of this regimen, a universal antidote is sometimes called a mithridate.
This fear of poisoning -- rational for an ancient king -- is said to have taken an ironic turn; when Mithridates was at last defeated by Pompey and in danger of capture by Rome, he is alleged to have attempted suicide by poison, but he couldn't take enough of the stuff to kill him and had instead to fall on his sword. In any case, vast improvements in the art of poisoning have been made since 63 BC, and tolerance against all known poisons cannot be built up in this manner; some, in fact, become more lethal on account of a cumulative effect. In other words, don't try this at home.