Ever since 1979, the U.S. Food and Drug Administration (FDA) has required that pharmaceutical companies put expiration dates on prescription and over-the-counter medicines.

That doesn’t mean your bottle of ibuprofen will go bad in the same way as, say, an expired carton of milk. The date that you see printed on a pill bottle is the date until which the medicine’s manufacturer will guarantee the drug’s safety and full potency. How long a drug actually remains safe and effective, however, is often a matter of debate.

Besides some medicines like insulin, nitroglycerin and liquid antibiotics, whose active ingredients are known to be less stable over time, many drugs might have a much longer shelf life than their packaging suggests.

But not everyone knows that, so poison control centers occasionally get calls from people who are concerned because they accidentally took expired medication, said Lee Cantrell, director of the San Diego Division of the California Poison Control System.

“The last time I checked, I haven’t seen any peer-reviewed documentation of expired medicine causing any problems in people,” Cantrell told Live Science. The effectiveness of medicines, however, may degrade over time, but there are few studies on the issue, he said.

That said, several years ago, Cantrell had a rare opportunity to examine an old stash of drugs — including antihistamines, pain relievers and diet pills — found in the back of a pharmacy.

Read more at Live Science