Objectives This study sought to assess the effectiveness and return on investment (ROI) of a multifaceted intervention aimed at improving antibiotic prescribing for acute respiratory infections in primary care. Design Large-sized, two-arm, open-label, pragmatic, cluster-randomised controlled trial. Setting All primary care physicians working for the Spanish National Health Service (NHS) in Galicia (region in north-west Spain). Participants The seven spatial clusters were distributed by unequal

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... andomisation (3:4) of the intervention and control groups. A total of 1217 physicians (1.30 million patients) were recruited from intervention clusters and 1393 physicians (1.46 million patients) from control clusters. Interventions One-hour educational outreach visits tailored to training needs identified in a previous study; an online course integrated in practice accreditation; and a clinical decision support system. Main outcome measures Changes in the ESAC (European Surveillance of Antimicrobial Consumption) quality indicators for outpatient antibiotic use. We used generalised linear mixed and conducted a ROI analysis to ascertain the overall cost savings. Results Median follow-up was 19 months. The adjusted effect on overall antibiotic prescribing attributable to the intervention was − 4.2% (95% CI: − 5.3% to − 3.2%), with this being more pronounced for penicillins − 6.5 (95% CI: − 7.9% to − 5.2%) and for the ratio of consumption of broad- to narrow-spectrum penicillins, cephalosporins, and macrolides − 9.0% (95% CI: − 14.0 to − 4.1%). The cost of the intervention was €87 per physician. Direct savings per physician attributable to the reduction in antibiotic prescriptions was €311 for the NHS and €573 for patient contributions, with an ROI of €2.57 and €5.59 respectively. Conclusions Interventions designed on the basis of gaps in physicians' knowledge of and attitudes to misprescription can improve antibiotic prescribing and yield important direct cost savings. Trial registration: Current Controlled Trials ISRCTN24158380. Registered 5 February 2009.