Objectives Unnecessary antibiotic prescribing contributes to antimicrobial resistance. A randomized controlled trial in 2014–15 showed that a letter from England’s Chief Medical Officer (CMO) to high-prescribing GPs, giving feedback about their prescribing relative to the norm, decreased antibiotic prescribing. The CMO sent further feedback letters in succeeding years. We evaluated the effectiveness of the repeated feedback intervention. Methods Publicly available databases were used to identify GP practices whose antibiotic prescribing was in the top 20% nationally (the intervention group). In April 2017, GPs in every practice in the intervention group (n=1439) were sent a letter from the CMO. The letter stated that, ‘the great majority of practices in England prescribe fewer antibiotics per head than yours’. Practices in the control group received no communication (n=5986). We used a regression discontinuity design to evaluate the intervention because assignment to the intervention condition was exogenous, depending on a ‘rating variable’. The outcome measure was the average rate of antibiotic items dispensed from April 2017 to September 2017. Results The GP practices who received the letter changed their prescribing rates by −3.69% (95% CI=−2.29 to −5.10; P<0.001), representing an estimated 124 952 fewer antibiotic items dispensed. The effect is robust to different specifications of the model. Conclusions Social norm feedback from a high-profile messenger continues to be effective when repeated. It can substantially reduce antibiotic prescribing at low cost and on a national scale. Therefore, it is a worthwhile addition to antimicrobial stewardship programmes.

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