Sorry, I just realized you were referring to mithridatism, and not a misspelling of metabolism.

Mithridatism is the gradual increase of a resistance against a poison by taking gradually increasing doses of that poison. Unfortunately, this would not be effective against Halon, partially because you cannot build up an immunity against the lack of oxygen. Furthermore, to say you cannot be poisoned because of mithridatism would mean that you have applied a technique that can take many years to every possible poison, which is impossible, seeing as how there are thousands upon thousands of poisons in the world, and this technique cannot be used to combat some of them.