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Do You Really Have a Penicillin Allergy?

Ask any doctor or pharmacist how many of their patients report a penicillin allergy, and you’ll find an astoundingly high number of self-reported penicillin allergic patients. As much as 10% of the population says they have a penicillin allergy. But this number appears to be falsely high.

A true penicillin allergy may consist of symptoms including rash, hives, itchiness and most concerning are the symptoms of anaphylaxis: swelling of the mouth or throat, difficulty breathing and a drop in blood pressure. The latter symptoms often come on quickly and can be fatal if not treated swiftly.

Symptoms such as stomach upset, nausea, vomiting, diarrhea or headache unaccompanied by the symptoms above usually are not related to allergy, but more due to actual side effects of the drug. If you have experienced these symptoms with a penicillin or related antibiotic, you probably do not have to avoid these antibiotics in the future.

Since many people may not know the difference between an allergy and a side effect, it is essential to tell your health care provider what types of symptoms you experienced when you had the “allergy”. This way the health care team will know how best to treat you if you have an infection. If it is not a true penicillin allergy, the medical team may not have to resort to a drug that kills a wide range of bacteria (increases resistance) or a drug that is much more expensive (increases wastage of money)!

Having said all this, what is the actual incidence of penicillin allergy? Surprisingly, it’s only 1-2% of all people! This is determined by skin testing, which can be performed if you are uncertain of the nature of your allergy, or if you don’t know if you are still allergic to a given drug.

There are many drugs related to penicillin:

Cloxacillin, amoxicillin, ampicillin, piperacillin are just a few close relatives that should be avoided if you have a true penicillin allergy.

Other more distant relatives include drugs called Cephalosporins, that begin with “Ceph-“ or “Cef-”, such as cephalexin, cefuroxime, cefixime, cefoxitin. If you are truly allergic to a penicillin, there is still a small (around 1-2%) chance of reacting to these drugs, depending on the specific drug. Other drugs that may cause an allergy if you are penicillin-allergic include meropenem, imipenem, and ertapenem. Cross reactivity between penicillins and aztreonam is rare.

In summary, if you are unsure whether you have a penicillin allergy, the chances low that you actually do. But the only way to be certain is if you’ve had symptoms of anaphylaxis to a penicillin, or have tested positive on a penicillin allergy skin test.