Penicillin allergy is a common finding in many patient charts.  Sometimes it can be difficult to figure out who, when and why this label was added to a medical record.  Sometimes patients themselves perpetuate the label, unwittingly reporting something a parent told them about a possible reaction as a child, or mistaking side effects for a reaction.  Either way, taking care of one patient with anaphylaxis is enough to strike fear into the heart of any prescriber.


A true allergy can be a dangerous issue and should, of course, be taken seriously.  However, up to 95% of people who report an allergy to penicillin are not actually allergic when they undergo formal allergy testing.


When patients for whom penicillin (or a member of the penicillin family) is the most appropriate choice report an allergy, providers are forced to make another selection.  This generally means moving to a broader spectrum antibiotic, which, unfortunately, puts patients at a higher risk for developing antibiotic resistant infections, like MRSA, and health care associated infections, like C. Diff.


A matched cohort study of 11.1 million patients in the UK from June 2018 (see reference below) sought to compare the risk of MRSA and C. diff in patients with a documented penicillin allergy.  After adjusting for risk factors for both MRSA and C. diff, patients with a penicillin allergy were found to have a 69% increased risk of infection with MRSA, and a 26% increased risk of infection with C. diff.


Less than 0.1% of patients who report a penicillin allergy receive formal testing.  As ED physicians, we can help by asking specific questions about reactions and updating charts, as well as providing patients with the allergy clinic contact info to find out for sure if their allergy is real.



Risk of meticillin resistant Staphylococcus aureus and Clostridium difficile in patients with a documented penicillin allergy: population based matched cohort study.  BMJ. 2018 Jun 27;361:k2400. doi: 10.1136/bmj.k2400.

May 2024