Are you allergic to penicillin? If so, are you sure about that?

It’s surprisingly common for people to wrongly think they have a penicillin allergy – and that misconception can be dangerous for their health.

Ten percent of all patients in the United States claim to have a penicillin allergy. Of those, 90 percent are not truly allergic and can tolerate the drug. That means millions of people take alternative antibiotics, which are more expensive and can put their health and potentially the health of others at risk. The solution is a simple allergy test.

A study in the British Medical Journal looked at six years’ worth of medical records for patients in the United Kingdom and found that those with a penicillin allergy had an almost 70 percent greater chance of acquiring a methicillin-resistant Staphylococcus aureus (MRSA) infection and a 26 percent increased risk of Clostridium difficile-related colitis (C. diff.). MRSA and C. diff. are major health risks worldwide. The study compared adults with a known penicillin allergy to similar people without a known penicillin allergy.

People labeled with a penicillin allergy are usually instead given broad-spectrum antibiotics, which may kill off more good bacteria along with the bad. This appeared to increase a patient’s risk of infection with MRSA or C. diff., which are common in our environment and can live without causing any problems on someone’s skin or gut. However, if a broad antibiotic kills off competing good bacteria, MRSA and C. diff. can thrive and start to cause problems.

“Penicillin-related drugs, that whole class . . . they’re very effective at killing, and they’re very targeted. So for some bacteria they’re still the best. Oldie but goody,” said Kim Blumenthal, lead author of the new study and assistant professor of medicine at Harvard Medical School.

“I have seen so many terrible, terrible outcomes” from C. diff. infections, Blumenthal said, including serious diarrhea, sepsis and death.

“All of us need to understand that antibiotic use is not a free ride, it carries a lot of risk,” said Paul Sax, clinical director of infectious diseases at Brigham and Women’s Hospital. He says the study adds to the “substantial body of evidence” which shows that a penicillin allergy has been linked to longer hospital stays and an increased risk of acquiring resistant infections.

Using non-targeted antibiotics can quickly breed resistant bacteria. “Not only is it harmful to the world and the general population . . . but it’s harmful to the individual patient. So the message to the public is that it could be dangerous to you or me,” said Helen Boucher, director of the Infectious Diseases Fellowship Program at Tufts Medical Center.