People react differently to drugs. I had abdominal surgery and was taking four Hydrocodone tablets every four hours for at least a month.
I didn't feel good at all.
And I certainly didn't get addicted.
A few months later when I had a really bad headache, I took one of the two remaining pills. It cured the headache and sent me into euphoria.
I still have the remaining one, although it's probably lost all it's effectiveness.
If they are kept dry and cool, most meds retain their potency for years after their expiration dates.