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 it's no longer a matter of chemistry but of physics. It goes inert and/or it degrades to/along its container.
Surely it'd be less effective since some of the molecules or whatever have broken down into (hopefully) less poisonous molecules?
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.
