system
Given the following post, respond as if you're commenting on Reddit:
human
What is happening at the atomic or molecular level that causes dry antibiotic and multivitamin pills to expire?

So apparently my Costco-sized jug of multivitamins expired in May 2009 (happy anniversary!) and my bottle of ibuprofen expires in 2013 some time, but what is happening to cause these pills to expire? Are the active ingredients degenerating? Are the pills becoming ineffective? Is this due to exposure to the atmosphere, or are they inherently unstable? 

Sorry if this is a dumb question, but the idea of "expired" drugs and vitamins never really made much sense to me. It seems as if the pills should be pretty inert, so what gives?
gpt
Nothing is perfectly inert - especially organic compounds. Many compounds are sensitive to water (Tylenol and Aspirin - acetaminophen and acetylsalicylic acid, respectively - contain amide bonds, which can slowly hydrolyze), others to oxidation (alcohol and unsaturated functional groups - virtually all the vitamins and drugs).

Most of these are not a huge concern, since the reaction rate is quite slow (_very_ slow, in fact), and steps _are_ taken to prevent decomposition (such as those silica pouches you find in the bottle, or the cotton ball at the mouth of the bottle). It's just that nothing is 100% effective - the silica doesn't suck up all atmospheric humidity, the seal on your bottle is not impervious.

So after a couple of years, or a decade or so, we can hardly say with a high degree of certainty how much of our active ingredient is left, or whether there are any toxic decomposition products formed. Other concerns include moisture build-up leading to fungal growths in the pills, etc.

Edit: Or, it could be a ploy by the big pharmaceutical companies to boost revenue.