When Maria Duggan developed a breast infection, she faced a choice no new mother would want to make: take an antibiotic that she’d been told could cause a severe allergic reaction, or one that would pass through her breast milk to her baby.

Like millions of Americans, Duggan, 32, had been diagnosed with a penicillin allergy as a child, but doctors believed she might have outgrown it. Still, the prospect of taking a drug that might send her into anaphylactic shock was scary.

“I knew there was going to be a risk in the decision and that risk was going to have to fall to me,” Duggan said.

After being prepared for the worst, Duggan had no reaction to the penicillin.

As it turns out, she is one of many Americans who’ve been told they have a penicillin allergy, but may been initially misdiagnosed or have grown out of it. Most doctors aren’t even aware that some 90 percent of those diagnosed with a penicillin allergy can actually tolerate the drug, according to a study presented recently at the annual meeting of the American College of Allergy, Asthma and Immunology.

Maria Duggan was diagnosed with a penicillin allergy as a child. When she developed an infection she faced the dilemma of taking a medication that might cause an allergic reaction, or one that might pass through her breast milk to her baby. Courtesy of Maria Duggan

“The typical situation is at a young age a child will get an infection and will get a course of penicillin and then will get a rash, or some other side effect, and get labeled as penicillin allergic,” said Dr. Elizabeth Phillips, a professor of medicine and pharmacology and director of personalized immunology, Oates Institute of Experimental Therapeutics at Vanderbilt University.

An estimated 25 to 50 million people in the U.S. who believe they are allergic may not truly be, said Phillips.

A simple skin test

Penicillin-related drugs are antibiotics with a long track record of safety. When people who’ve been labeled as allergic to penicillin need antibiotics for serious infections, "they may get a second antibiotic that has more side effects or is less effective for the condition they need treated,” Phillips said.

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The solution for many is a simple, two-step test, followed, as needed, by a low-dose oral penicillin, taken under a doctor's observation.

"The whole process takes about three hours and then we can say they’re free to take penicillin in the future,” said Phillips

Hollis Heavenrich-Jones tested her son, Spencer's reaction to penicillin before he needed it.

Spencer Heavenrich-Jones was diagnosed with a penicillin allergy as a baby. A later skin test show no allergic reaction. Courtesy of Hollis Heavenrich-Jones

The child was diagnosed with a penicillin allergy when he was a baby, but Heavenrich-Jones wanted to be sure.

“He had his one and only ear infection and they gave him penicillin and he broke out in a sort of head-to-toe rash,” Heavenrich-Jones said. “It went into his chart and it’s been there ever since.”

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Spencer underwent a skin test and took an oral dose. Both revealed no allergy, a "huge relief" to his mother.

An earlier version of this story stated Spencer had a reaction to a control substance during a test. It has been updated to reflect that his skin tests were negative.