Every now and then you reach into the bathroom cabinet to find that some pill or ointment you where planning to use has gone past its expiration date.
I am interested whether this expiration date has any value. In a short search I found this article from Harvard medical school stating that some test found that 90% of all drugs where still perfectly active 15(!) years after the expiration date. The article then continues to give some vague statements on possible reasons for the expiration date (including financial reasons).
What I am wondering is whether there would be any chemical reasons for drugs to become less effective or even become harmful over time, which could warrant the existence of an expiration date?
As a specific example, in case it is difficult to answer this question in general, let's take an ointment meant to treat muscle pains which use Benzydaminehydrochloride as an active component.