What a great question! Maybe at some point it achieves sentience- that should be well past the use by date even for poison, one would hope. I was about to open a can of beans from 1991 the other day. It may have been poison once, then gone past that point and become a completely contained eco-system until it exploded all over my kitchen.
Predictably my dog tried to lick it up but my son came to the rescue and said, "I'll call for pizza," as he led her away into his room. Took me a good half-hour to clean beans off the ceiling and every other surface. There were 24 year old beans in my hair when my husband got home.
Husband "Are we having beans for dinner?"
Me "No, pizza."
Husband "Bean pizza?"
As for poison, I know some of it at least has a really long shelf life- thousands of years. Some gets more poisonous past the expiry date. Some gets less effective. I know of none that becomes good for you - like breaks down to become a full days supply of vitamins and minerals.