"King of Pontus on the southern shore of the Black Sea, Mithridates VI (d. 63 BC) concocted one of the most well-known antidotes in antiquity (possibly with the help of his court physician Crateuas). Experimenting with different formulations and trying them out on condemned prisoners, he compounded various antidotes to produce a single universal one, which he hoped would protect him against any poison. A hundred years after the death of Mithridates, Celsus recorded the formulation, which comprised thirty-six ingredients, all of which are derived from plants, except for honey to mix them and castor to enhance the aroma. The concoction is estimated to have weighed approximately three pounds and to have lasted for six months, taken daily in the amount the size of an almond."There are additional relevant quotations here, along with discussion which includes the report that when Mithridates faced military defeat and tried to commit suicide by poison, he was unable to do so.
"But the most famous antidote is that of Mithridates, which that king is said to have taken daily and by it to have rendered his body safe against danger from poison. It contains costmary 1.66 grams, sweet flag 20 grams, hypericum, gum, sagapenum, acacia juice, Illyrian iris, cardamon, 8 grams each, anise 12 grams, Gallic nard, gentian root and dried rose-leaves, 16 grams each, poppy-tears and parsley, 17 grams each, casia, saxifrage, darnel, long pepper, 20.66 grams each, storax 21 grams, castoreum, frankincense, hypocistis juice, myrrh and opopanax, 24 grams each, malabathrum leaves 24 grams, flower of round rush, turpentine-resin, galbanum, Cretan carrot seeds, 24.66 grams each, nard and opobalsam, 25 grams each, shepherd's purse 25 grams, rhubarb root 28 grams, saffron, ginger, cinnamon, 29 grams each. These are pounded and taken up in honey. Against poisoning, a piece the size of an almond is given in wine. In other affections an amount corresponding in size to an Egyptian bean is sufficient."
Celsus, De Medicina (V.23.3)
I find it interesting that the antidote doesn't contain any "terra sigillata" or clay materials, which would have been useful against heavy metals. But eating clay on a daily basis raises the risk of inducing zinc deficiency.
Finally, a link for those who like me don't know where "Pontus" was/is (the name is derived from the Greek word for the Black Sea).