The requirement for an expiration date stems from the Johnson administration. The idea was to ensure that consumers and patients had some reasonable certainty that foods and medicines were still safe and fully effective when used.
Hence, medications are guaranteed to be over 90% of original potency by date of expiration. (Assuming you haven't gotten them wet, or stored them outside of recommended storage temperatures- leaving them in your car, for example.)
It turns out, there is quite a lot of interest in how potent your medications are after the date of expiration.
For example, the military has large stockpiles of emergency supplies including medications for a variety of reasons. The FDA (your taxpayer dollars) administers the military's Shelf-Life Extension Program (SLEP), to test extended shelf-life of stockpiled medicines. 122 different medications stored in 3,005 lots were studied:
- 88% of them extended past 1 year (were felt to be effective and usable)
- average extension was 5 years
- antibiotics that were studied included:
- Ciprofloxacin- extended 55 months
- Amoxicillin- extended 23 months
- doxycycline- extended 50 months
It should be stressed that unlike foods, medications do NOT become toxic with prolonged shelf-life.
Also, this information relates to dry medications. Eye drops, inhalers, injectables are an open question. Certainly, the EpiPen is basically inert within a month or less after expiration!
So, in a nutshell, many solid medications are probably active for up to around 5 years after expiration.