Data on the prevalence and incidence of adverse reactions to antibiotics in outpatient populations are rare. These events are commonly called “allergy” when noted in the medical record.

Determine the prevalence and incidence of allergy, as recorded in the medical record, to the most commonly used antibiotic classes in a large outpatient population using health care in the United States during 2007.

Data for drug allergy and antibiotic use were extracted from the electronic health records of 411,543 patients cared for by Kaiser Permanente in San Diego County who had at least one outpatient visit during 2007. Outpatient antibiotic utilization data was obtained for each year between 1995 and 2007. Penicillins, sulfas, cephalosporins, tetracyclines, macrolides, and quinolones were the classes of antibiotics evaluated.

Antibiotics account for a majority of drug allergy entries. Antibiotic classes with higher historical use have higher allergy prevalence. Female patients use more antibiotics than males, and have higher allergy prevalence rates for all classes of antibiotics. There is a steady increase in antibiotic allergy prevalence with aging for both sexes. Females have higher allergy incidence rates for all classes of antibiotics. Antibiotic allergy incidence in female patients is highest for sulfas, 3.4%, compared with 1%-1.5% for all other classes of antibiotics. Antibiotic allergy incidence in males also is highest for sulfas, 2.2%, compared with 1.1% for penicillins and 0.5%-0.6% for all other classes of antibiotics.

Female sex, use, and increasing age are the primary factors that account for higher antibiotic allergy prevalence. Antibiotic allergy incidence is highest with sulfa class antibiotics.