Hamilton, Ontario—Pharmacists might be surprised to hear that the vast majority of people who report a penicillin allergy actually don’t have one.

The report in the Canadian Medical Association Journal (CMAJ) argues that nine out of 10 people who believe they’re allergic to the antibiotic either aren’t allergic or have only some intolerance, and eight of 10 people who had an allergic reaction to penicillin a decade or more ago now can safely use the drug.

According to the article written by two McMaster University physicians, the quickest way to find out if a true penicillin allergy exists in the most likely cases is referral to an allergist for testing.

While penicillin allergy is commonly reported, 90% of the time a patient actually can tolerate penicillin. The authors note that, even though about 10% of people report a penicillin allergy, the overwhelming majority are not truly allergic because intolerances are sometimes mislabeled as allergies. In addition, even in those who were allergic at some point, allergies tend to wane over time.

To that point, the authors emphasize that penicillin allergy is lost over time, resolving in 50% of people over 5 years and 80% over a decade. “Those who had reactions more than 10 years ago are unlikely to still be allergic and should be tested before given penicillin,” the authors write. “If there is a strong indication for antibiotics, an allergist physician should be consulted about therapy.”

The article also suggests that labeling patients as having penicillin allergies is bad for them and the healthcare system. Their reasoning is that more costly and less effective second-line and broad-spectrum antibiotics must be used, and those have a significantly increased risk of infections such as methicillin-resistant Staphylococcus aureus and Clostridium difficile, according to the report.

The authors advise healthcare professionals to evaluate whether patients who suspect penicillin allergy need to be seen by a specialists. The article notes that a side effect such as nausea after using penicillin should not be considered a penicillin allergy. In addition, patients without a personal history of a penicillin allergy or who have tolerated penicillin in the past, do not need to avoid penicillin.

While rare, severe allergic drug reactions causing hospitalization due to widespread skin blistering, organ failure, and/or joint swelling should be evaluated by a specialist, however, and those patients should strictly avoid use of penicillin before that. The authors explain that true immediate allergic reactions cause rapid-onset hives, lip and face swelling, and anaphylaxis. Patients who have had these type of reactions or think they might have should be evaluated by an allergy specialist, the article states.

The authors decry the underuse of allergy referral and testing, even though it is accurate, fast, and cost-effective. 

“Allergy testing over one to two hours using a combination of skin and challenge testing by trained personnel has been shown to be safe and effective for children and adults close to 100 per cent of the time,” the reviewers write.

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