To complicate matters a bit further, Pliny didn't originate this idea. Rather, he was detailing the aftermath of the third Mithridatic War, a conflict in which Rome conquered the Persian Kingdom of Pontus. After Rome defeated Mithridates, the ruler of Pontus, the Roman general Pompey searched through the fallen enemy's things. In Mithridates' possessions, Pompey found an antidote that read, via
Mental Floss, "Take two dried walnuts, two figs, and 20 leaves of rue; pound them all together, with the addition of a grain of salt." The concoction was said to protect the drinker from poisoning.