Patients with a documented penicillin allergy have an increased risk of new methicillin resistant Staphylococcus aureus (MRSA) and Clostridium difficile (C difficile) that are modifiable, in part, through changes in antibiotic prescribing. This according to a population-based matched cohort study that included 301,399 adults without previous MRSA or C difficile enrolled in the Health Improvement Network database: 64,141 had a penicillin allergy with 237,258 comparators matched on age, sex, and study entry time. The primary outcome was risk of incident MRSA and C difficile; secondary outcomes were use of β lactam antibiotics and β lactam alternative antibiotics. Researchers found:

• Among 64,141 adults with penicillin allergy and 237,258 match comparators, 1,365 developed MRSA (442 with penicillin allergy and 923 comparators) and 1,688 developed C difficile (442 with penicillin allergy and 1,246 comparators) during a mean 6.0 years of follow-up.
• Among patients with penicillin allergy, the adjusted hazard ratio for MRSA was 1.69 and the C difficile was 1.26.
• The adjusted incidence rate ratios for antibiotic use among those with penicillin allergy were 4.15 for marcolides, 3.89 for clindamycin, and 2.10 for fluoroquinolones.
• Increased use of β lactam alternative antibiotics accounted for 55% of the increased risk of MRSA and 35% of the increased risk for C difficile.