You see the stamp on virtually every bottle of prescription or over-the-counter drug: the day, month, or year when it’s expected to expire. An old container of aspirin might have gone "bad" months ago—is there harm in taking it? Drugs aren’t like milk, right? Isn’t this just forcing you to spend money on a new supply?

No. And here’s why. According to the Food and Drug Administration, drugs that have outlived their shelf life offer no guarantee that the compound will be as safe or as effective as it was when it was still considered viable. Chemical compositions can change over time, rendering a medication less potent than expected.

Expiration dates (which were introduced in 1979) are based on the FDA’s high standards for stability and take a very conservative approach: Studies have shown a drug can often retain most of its potency for a year or more after the expiration date when ideal conditions—avoiding humidity, for example—are met.

But potency shouldn’t be confused with safety. No human studies have been conducted that have measured the effectiveness of expired drugs, and ethical considerations virtually guarantee no such research will ever be conducted.

Because storage, compositions, and other variables can vary widely, the best idea is to discard expired medications. (That’s especially true in the case of some medications such as EpiPens, which are carried by those who need to arm themselves against life-threatening anaphylaxis, and where a loss of strength can carry serious consequences.) If a drug comes with two expirations—one from the supplier and one from your pharmacist—pay attention to the shorter of the two. And if you insist on using the meds anyway, recognize that they may not do as good a job at curing what ails you.