It’s a common enough question in any doctor’s office: “Do you have any drug allergies? Penicillin maybe?” Up to 15 percent of patients in the US will say they’re allergic to penicillin. That’s tens of millions of people. But studies have shown that when those patients are challenged with tests, more than 95 percent turn out to be dead wrong.

It might not seem like a big deal, but it is. Penicillin and its close relatives are often first-line drugs. When they’re skipped due to a phantom allergy, doctors land on harder drugs—antibiotics that can be less effective, have more side effects, and spur resistant infections. Indeed, some studies have found that people with penicillin allergies have higher odds of carrying drug-resistant bugs.