While 10% of people in the United States report being allergic to penicillin, 9 in 10 are not. However, this can be difficult to scratch from medical records.

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New research suggests that many adults who used to be allergic to penicillin in their childhood may have outgrown the allergy.

So concludes one of three studies presented at a scientific meeting in Houston, TX, shedding fresh light on the antibiotic.

Alexander Fleming discovered penicillin by chance in 1928 when he noticed that penicillin fungus was a potent antibacterial. Doctors first used this now common antibiotic as medicine in the 1940s, finally giving doctors a weapon against deadly infections.

Penicillin went into mass production in the U.S. during World War II, proving its worth on the battlefield where, until then, infection had caused more deaths than war wounds. Today, doctors use this antibiotic to treat a wide range of infections.

However, if an adult or child has ever had a reaction to penicillin, and a doctor has told them that they are allergic to it, the chances are that this "fact" is still on their medical records.

So say researchers who have found that 9 in 10 people in the U.S. who think that they are allergic have outgrown the allergy or were never allergic in the first place.

The researchers add that even when people test negative to a penicillin allergy at a later date, the allergy often remains on their medical and pharmacy charts.

"Our study found that of the 52 patients who tested negative to penicillin and were interviewed, 98% understood they were not allergic to penicillin," says lead author, Dr. Sonam Sani, an allergist, immunologist, and fellow of the American College of Allergy, Asthma, and Immunology (ACAAI).

"Of those, 29% still had a penicillin allergy label in their electronic medical record, and 24% still carried the label in their pharmacy records."

The study was one of three into penicillin allergy presented at the ACAAI Annual Scientific Meeting in Houston. The other two examined gender and penicillin allergy, and childhood testing of the allergy.

Why allergy testing is important

In the first study, researchers conducted follow-up interviews with people who had tested negative to a penicillin allergy and reviewed their electronic medical records and pharmacy records.

Four people continued to report a penicillin allergy to new healthcare providers and avoid the antibiotic.

Some of the issues with using broad-spectrum antibiotics as an alternative to penicillin for those deemed allergic include higher costs, lower efficacy, and the risk of boosting antibiotic resistance, which is a global health threat.

People must get tested by an allergist to confirm their allergy status.

"Once patients have tested negative for penicillin allergy, there needs to be collaboration between patients, physicians, and pharmacists to make sure their records reflect it's safe to take penicillin again," says Dr. Sani.

"Anyone who has tested negative for penicillin allergy should have confidence in their test results and be able to use penicillin antibiotics for future infections."

Women more likely to be allergic than men

In the second study, the researchers found that women were four times more likely to have an allergy to penicillin than men.

The study, which looked at over 30,000 penicillin allergy tests that took place between 2001 and 2017, examined 3x3 and 5x5-millimeter (mm) skin wheals. Researchers found the 3x3-mm wheal showed a notable difference between men and women in terms of penicillin allergy.

"Our study suggests women have a higher rate of penicillin allergy than men," says study co-author Dr. Miguel Park, allergist, and ACAAI member. "Of the 329 people with a positive skin test, 298 (91%) were female, and 31 (9%) were male."

"Further studies will need to be done to verify these results, but getting tested for penicillin allergy is clearly worthwhile for those who have the diagnosis in their medical chart," Park concluded.

Oral challenge safe in low-risk children

Traditionally, doctors test children for a penicillin allergy with a skin test. If the test is negative, doctors typically follow it up with an oral dose of penicillin.

However, the third study presented in Houston found it was safe and effective to leapfrog straight to an oral challenge with amoxicillin, which is a type of penicillin.

During the study, 54 children labeled as having a penicillin allergy received an oral challenge of amoxicillin.

"Of those, 100% passed the challenge, and none developed any reactions," says study co-author, Dr. Jennifer Shih, allergist, and ACAAI member. "None of the children had ever had a severe reaction to amoxicillin, so all were low-risk for the challenge."

"All the children were able to have the allergy label removed from their charts. Our study suggests that a direct oral challenge without preliminary testing in low-risk children is a safe, effective method to rule out penicillin allergy."

Dr. Jennifer Shih