@ sinkingship
It depends on the poison. There are some you can use the method of mithridatism (i.e. what is going on in the story), but there are certainly some poisons that mithridatism simply doesn't work on. It really depends on the substance in question, however, there's proof in day to day life that mithridatism works, after all, it's how your body builds a tolerance for the poison called alcohol.