In the case of tetracycline antibiotics the degradation products can damage the kidneys and cause Fanconi syndrome. So in that case as a medicine for people it becomes poison, as a poison for bacteria it becomes safer.

Not necessarily.

Aspirin yes, paracetamol no.

Some medicine also just becomes less effective, not more dangerous.

If you wait long enough does poison expire and become medicine again tho?

Someone hire this guy! Nobel material.

Presumably yes, and for historical context look to Napoleon Bonaparte. It's presumed that when he attempted suicide the mercury cyonide had "expired" (humidity and water got in) and that's why his suicide failed. I still wouldn't go around eating expired poison to prove a point though.