Not everyone likes opiates. My Jenny was recently prescribed some for her osteoarthritis. It didn't help with the pain at all and it fucked up her sleep and just made her feel queasy. She tried a couple and now she's done with it.
For me, once upon a time, that bottle of pills would have been a godsend and I would have heard a chorus of angels whenever I opened the lid.
But for her it is worse than nothing.
My ex was like that too. No thanks to opiates. She would get them and take a couple and then they would sit patiently marching towards their expiry date in the medicine cabinet.
I dunno. Pretty much all drugs have an expiry date. In this part of the world anyway. It's there and there is a legal requirement for pharmacists and sellers to heed it.
Now if you're asking me for an explanation of chemical changes and whatnot, I can't help you. I can tell you I have taken many drugs including opiates well past the expiry date and they worked just the same.
The potency of the substance wears out over time. The date is put on the pills so the user knows that they won't be getting the "prescribed" dose from their "doctor." if taken after expiry. As to the amounts and significance to whatever chemical compounds you've taken and it's time after the expiration date I have no idea.