Heat and steam from your shower or shave can rob medicine of its potency long before the drug's expiration date. Angela Cappetta/Getty Images hide caption

Heat and steam from your shower or shave can rob medicine of its potency long before the drug's expiration date.

Most of us have reached for a painkiller, at one time or another, only to discover the date on the label shows it has expired. But what does an "expiration" date on medicine really mean? Is it dangerous if you take it anyway? Less effective?

It turns out that date stamped on the label actually means a lot. It's based on scientific evidence gathered by the manufacturer showing how long the drug's potency lasts. Companies expose their medications to different environments, different temperatures and humidity levels to see just how long it takes for the medication to degrade to the point that its effectiveness is compromised.

The general rule, says pharmacist Mike Fossler, with the American College of Clinical Pharmacology, is that once a drug is degraded by 10 percent it has reached "the end of its useful life." If you take it months or even years past the expiration date, it's unlikely to do you any harm, he says; it just might not do you much good.

That may not be a big deal if you're treating a headache, but if you're fighting a bacterial infection with antibiotics like amoxicillin or ciprofloxacin, for example, using less than fully potent drugs could fail to treat the infection and lead to more serious illness.

Pharmacist Mohamed Jalloh, a spokesman for the American Pharmacists Association, says there's an even bigger reason not to rely on old drugs: antibiotic resistance. When you inadvertently "underdose" yourself by taking antibiotics that aren't full strength, he says, you run the risk that the bacteria you're battling will figure out not only how to defeat this weakened drug, but other antibiotics, too.

At least 23,000 people each year in the U.S. die from infections that have become resistant to antibiotics, according to the Centers for Disease Control and Prevention.

"If your medicine has expired, don't use it," concurs Ilisa Bernstein, deputy director of the office of compliance in the Food and Drug Administration's Center for Drug Evaluation and Research.

That goes for over-the-counter drugs, as well as prescription meds. Check the expiration date before even buying those pain relievers or allergy tablets, some pharmacists advise the same way you check your milk. Buy the one with the date that's furthest away.

"Once the expiration date has passed," Bernstein says, "there is no guarantee that the medicine will be safe and effective."

Of course, even new drugs can quickly lose potency if they're not stored properly. Get those pills out of the bathroom "medicine cabinet" now, pharmacists say. The steam from your shower or shave kills pills fast.

"Medicines like the kind of environment that people like a little dry and not too hot or cold," Fossler says. And, of course, don't take medication to the beach or leave it in a hot car. Like humidity, heat degrades a medicine's active ingredients.

Some medications are more vulnerable than others, so check the label. Insulin, certain immunotherapy drugs, and some children's pain relievers and cold remedies require refrigeration and protection from light.

And compared to capsules and tablets, "liquids are not as highly preserved," says Barbara Kochanowski, a scientist with the Consumer Healthcare Products Association. Liquid drugs can more easily become contaminated with bacteria and fungus.

Anytime you see a change in the color, odor or consistency of a drug such as a cream turning into a runny solution consider it a red flag, Kochanowski says, and consult your pharmacist. It's probably time to toss that medication.

Some drugstores, hospitals with pharmacies, drugmakers and drug-treatment centers have been authorized by the federal government, in recent years, to serve as "drug-take-back" sites for some drugs that are expired, or no longer needed. You can check the FDA and Drug Enforcement Agency websites for their latest guidance on the safest ways to dispose of various drugs.

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Heat, Humidity And Aging Make Medicine Less Potent : Shots - NPR - NPR