...medicines were effective many years after the expiration dates

Agree, with a caveat or two

It depends on large part on the storage conditions. Meds in a first-aid kit in the trunk of my car (epi-pens, Narcan) I try to replace more-or-less as they expire, i.e. within a year or two of the expiration date. It gets hot in there.

Most of my backup meds, however, are kept in the dark, specifically in a military surplus ammo can -- the type with a waterproof seal -- in the basement. There they are in the dark in a perpetual 50-70 degrees or so with controlled humidity. Those meds I anticipate to stay at or near their original potency more or less forever.

(There are a couple of exceptions, as the active molecules spontaneously degrade over time. Sublingual nitroglycerin (unstable angina) is one. For some reason, tetracycline (older antibiotic) and possibly dilantin (seizures) stick in my head from my med school pharmacy course of, uh, 40+ years ago as another two. I'm not sure of this last statement, however. Regardless, these aren't meds I keep in the kit).

-- sutton