According to the Harvard Health Letter, an expiration date is a legal requirement that has been imposed by the FDA since 1979. It is meant to be the date until when "the manufacturer can still guarantee the full potency and safety of the drug.” However, research shows that 90% of more than 100 types of medication, both over-the-counter and prescription, are fine to use even 15 years after they have supposedly expired.
A report from Medscape claims that expiration dates don't tell you how long the medication “is actually ‘good’ or safe to use”, and in fact, many medical authorities claim that it's okay to take most drugs well after they've supposedly expired.
The JAMA study went well past 15 years, with the researchers analyzing different medications with 14 distinct active ingredients in them that had expired anywhere between 28 and 40 years ago. Their findings revealed that most of the active ingredients were still at least 90% effective, which is considered to be the minimum potency acceptable. The only drugs which fell below this threshold were aspirin, phenacetin (a painkiller), and amphetamine (for ADHD and narcolepsy).
The former director of the FDA’s testing program, Francis Flaherty, said that “expiration dates put on by manufacturers typically have no bearing on whether a drug is usable for longer.” He also said that these dates were typically used for “marketing, rather than scientific, reasons… it’s not profitable for them to have products on a shelf for 10 years. They want turnover.”