Background: Prescribing medicines for older adults in care homes is known to be sub-optimal. Whilst trials testinginterventions to optimise prescribing in this setting have been published, heterogeneity in outcome reporting hashindered comparison of interventions, thus limiting evidence synthesis. The aim of this study was to develop a coreoutcome set (COS), a list of outcomes which should be measured and reported, as a minimum, for all effectivenesstrials involving optimising prescribing in care homes. The COS was developed as part of the Care HomesIndependent Pharmacist Prescribing Study (CHIPPS).Methods: A long-list of outcomes was identified through a review of published literature and stakeholder input.Outcomes were reviewed and refined prior to entering a two-round online Delphi exercise and then distributed viaa web link to the CHIPPS Management Team, a multidisciplinary team including pharmacists, doctors and PatientPublic Involvement representatives (amongst others), who comprised the Delphi panel. The Delphi panellists(n = 19) rated the importance of outcomes on a 9-point Likert scale from 1 (not important) to 9 (critically important).Consensus for an outcome being included in the COS was defined as ≥70% participants scoring 7–9 and <15%scoring 1–3. Exclusion was defined as ≥70% scoring 1–3 and <15% 7–9. Individual and group scores were fed backto participants alongside the second questionnaire round, which included outcomes for which no consensus hadbeen achieved.Results: A long-list of 63 potential outcomes was identified. Refinement of this long-list of outcomes resulted in29 outcomes, which were included in the Delphi questionnaire (round 1). Following both rounds of the Delphiexercise, 13 outcomes (organised into seven overarching domains: medication appropriateness, adverse drug events,prescribing errors, falls, quality of life, all-cause mortality and admissions to hospital (and associated costs)) met thecriteria for inclusion in the final COS.Conclusions: We have developed a COS for effectiveness trials aimed at optimising prescribing in older adults in carehomes using robust methodology. Widespread adoption of this COS will facilitate evidence synthesis between trials.Future work should focus on evaluating appropriate tools for these key outcomes to further reduce heterogeneity inoutcome measurement in this context.