I’ve been a practicing allergist and immunologist in Maine for almost 10 years, and recently my mother-in-law came in to see me about a penicillin allergy. She had heard me talk about how we can now accurately test for this common condition.

Her tests came up negative. She had spent her whole life thinking she was “deathly” allergic to penicillin, only to find out that she either had been misdiagnosed, or she had outgrown it. This common clinical scenario makes me think how many others are out there with the same misdiagnosis. Turns out, there are quite a few.

Up to 10 percent of the U.S. population think that they are allergic to penicillin, but recent studies show that a small fraction of those people truly are. For most, the label “penicillin allergic” results from mistaking an adverse reaction, such as vomiting or diarrhea, for an allergy. It’s also possible, especially in children, to confuse an unrelated viral rash as being caused by penicillin.

Some may have been allergic to penicillin in the past, but the allergy has gone away over time. Indeed, studies show that up to half of all patients who had immediate reactions to penicillin will no longer have allergic antibodies to penicillin after five years, and up to 80 percent will no longer have allergic antibodies to penicillin after 10 years.

When you take all of this into consideration, a relatively small number of people are actually allergic to penicillin.

So why are so many people mislabeled in the first place? For years there were no reliable, standardized testing methods available, and so the diagnosis was based solely on the history of reaction.


And with the credo of “first do no harm” entrenched in all physicians, the general tendency was to err on the side of caution and recommend avoidance. This approach was justified, since a legitimate penicillin allergy can result in anaphylaxis: a potentially fatal (albeit rare) severe chain reaction within the body.

Consequently, avoidance of penicillin and related antibiotics has been the dogma for several decades, in part because of the readily available multitude of antibiotic alternatives.

But this landscape is changing. Thanks to the relatively recently Food and Drug Administration-approved commercially available penicillin testing materials, we know that a clinically diagnosed penicillin allergy is often inaccurate.

But you might be wondering, “So what’s the big deal? I’ve been avoiding penicillin and getting alternative antibiotics over the years without issues.” Well, it turns out that avoidance of penicillin often restricts doctors to prescribing medicines that can be less effective, more expensive and possibly have more side effects than penicillin.

As well, the use of these alternative antibiotics encourages the widespread development of resistant organisms, and dramatically increases U.S. health care costs. For these reasons, elective testing is gaining favor.

Penicillin allergy testing involves a simple, minimally invasive and reliable office-based test done by a board-certified allergist. If the test is negative, you will be given an oral dose of a penicillin drug, and monitored in the allergist’s office for 60 to 90 minutes. If you are negative on skin testing and do not react to an oral challenge, you may safely take penicillin in the future. A positive skin test or adverse reaction to the oral challenge would confirm you should not take penicillin.


Penicillin allergy is the most common drug allergy. For those not allergic, penicillin is safe to use during pregnancy and breastfeeding, well tolerated in children and very economical. It is therefore beneficial to find out if you are indeed penicillin allergic. Of the approximately 27 million Americans reporting penicillin allergy, fewer than 0.1 percent undergo testing.

With the increased use of electronic medical records, drug allergies are a part of your medical record, likely to be shared with all doctors and hospitals. Once you’ve been labeled “penicillin allergic,” you won’t receive penicillin or a related drug.

So if you are wondering if you are allergic to penicillin, consider seeing an allergist for testing, since it is usually not possible to determine penicillin allergy based solely on patient history.

An allergist can determine if you are allergic, or if the label can be removed from your chart. (To find a board-certified allergist near you, visit acaai.org/locate-an-allergist.) It’s especially important to find out before you are in urgent need of a penicillin drug to treat a serious illness.

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