Most prescription drugs "expire" after two to three years—or so we've been told.

Department of Defense - defense.gov

But researchers have started challenging the idea that medicine becomes ineffective or dangerous when it exceeds its designated life span. Not only does that appear to be untrue for the bulk of prescription medications, but misleading expiration dates come at a cost to everyone.

"We didn't have any idea that some of the products would be so damn stable—so robustly stable beyond the shelf life," Ajaz Hussain, a scientist who tested drug expiration dates for the FDA, told NPR. "It's a shame to throw away good drugs."

How do we know that drugs last longer than the expiration date printed on the prescription bottle?

For one, the Food and Drug Administration (FDA) has conducted routine testing of the different drugs that the federal government stockpiles in the event of an emergency, in order to determine whether the expiration dates could be extended. In 2006, FDA researchers released a study that analyzed 122 drugs in those stockpiles: 88 percent were "extended beyond their original expiration date" for an average of 5.5 years.

More recently, researchers at the University of California, San Diego, analyzed the potency of 14 drugs that were discovered in a forgotten pharmacy closet. Tests revealed that 12 out of 14 of the drugs had retained their potency—even 30 to 40 years after they'd expired.

Why does this matter?

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Tossing out expired-but-not-really-expired drugs prescription drugs means losing money. Not only does it represent waste for hospitals and pharmacies that are required to dispose of them, but that waste ultimately translates into higher health care costs for consumers because insurers covering prescription drugs make up for the extra spending though higher premiums and deductibles.

Based on the value of expired drugs that certain hospitals have reported tossing out, ProPublica estimated that hospitals dispose of $800 million worth of drugs each year. And that figure doesn't include the value of prescription drugs that are disposed "at long-term-care and retail pharmacies and in consumer medicine cabinets."

[h/t ProPublica]