[mattmanser]

I used to work in QA for a major drug company where we'd store the drugs for years at different temperatures and humidity, and every 3/6/12/18/24/48/60 months we'd test them in spectrometers, as well as testing for clumping in powders, colour changes, etc. One of our jobs was to make sure they didn't turn into anything nasty, but it was mainly making sure they stayed stable.

For all day-to-day drugs it's really a precaution rather than anything else. Usually they just become a little less effective by becoming less concentrated, but most of the ones I worked with even the 50/80(?) ones were still fine years later (stored at 50 degrees celsius, 80% humidity (I think? Was 20 years ago)). We had a special warm room where all the samples were kept.