This basically depends on the kind of poison. The name "poison" has many many substances under it. What exactly happened to it over time is another question. Expiring happens due to chemical reactions over time (well that's one of the reasons), and the question is what did it react with.
I mean, if you eat sodium you'll burn alive, and if you breath in chlorine, you'll die (unless it's in very small amounts). If they react together you'll get salt... Yeah, the stuff you have on your dinner table, and aside from the discussion if salt is good for you or not, it's not toxic anymore and suitable for consumption.
Of course, there are also poisons that become more poisonous due to the reactions they undergo.
So from scientific point of view, there is no correct answer, as there are too many uncertainties.
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