Penicillin Allergy

My patients are smart and knowledgeable, but I find that many mistakenly believe that they are allergic to medicines, and in particular, penicillin. This belief might be based on a distant memory, or on something they were told by their parents. Others believe that the experience of an upset stomach, nausea, or other side effect indicate an allergy. Still others may have experienced a rash or other symptom that was actually due to the illness, and not to the medicine. Still, these numbers are surprising: as many as 100 of every 1,000 patients believe that they are allergic to penicillin, but only about five of that 1,000 actually are!

Does this matter? Usually not. Your doctor can often find an appropriate alternative antibiotic that will work just fine. (Note that in the case of penicillin, it is likely that if you react adversely to one form of penicillin, you will likely react to other forms as well.) But what about when a penicillin-related medicine would be a better choice than those alternatives, or is the only choice? It may well be worth finding out if you do currently have such an allergy.

When I take a patient’s history, I ask what the reaction was, how many days or hours had passed between starting the medication and experiencing the reaction, whether the medicine has been tried again since that initial episode, and whether there were other associated symptoms.

People who are allergic to penicillin do indeed often develop a rash. The rash can develop after the first or second dose, or it might first show up several days after completing the course of antibiotics. An immediate (within an hour) allergic reaction can include rash, hives and swelling. Occasionally, anaphylaxis occurs, an immediate reaction and a medical emergency which can include difficulty breathing, dizziness, vomiting and loss of consciousness. If hives or anaphylaxis do not occur, then an allergy is not likely.

Allergy testing can answer the question. Starting with a skin test, the skin of the forearm is pricked with different components of penicillin. If the results are negative, then a small amount of penicillin is placed under the skin. If that is negative, there may be a penicillin oral challenge: The patient takes a dose of amoxicillin and is monitored for one hour. If there is no allergic reaction, you can safely assume that there is no penicillin allergy.

The next time you see your doctor, feel free to raise this conversation. You can get a conclusive answer regarding a possible allergy to penicillin.