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OBJECTIVE To estimate the proportion of healthcare interventions tested within Cochrane Reviews that are effective according to high-quality evidence. STUDY DESIGN AND SETTING We selected a random sample of 2428 (35%) of all Cochrane Reviews published between 1 January 2008 and 5 March 2021. We extracted data about interventions within these reviews that were compared with placebo, or no treatment, and whose outcome quality was rated using Grading of Recommendations Assessment, Development and Evaluation (GRADE). We calculated the proportion of interventions whose effectiveness was based on high-quality evidence according to GRADE, had statistically significant positive effects, and were judged as beneficial by the review authors. We also calculated the proportion of interventions that suggested harm. RESULTS Of 1567 eligible interventions, 87 (5.6%) had high quality evidence on first-listed primary outcomes, positive, statistically significant results and were rated by review authors as beneficial. Harms were measured for 577 (36.8%) interventions, 127 of which (8.1%) had statistically significant evidence of harm. Our dependence on the reliability of Cochrane author assessments (including their GRADE assessments) was a potential limitation of our study. CONCLUSION Most healthcare interventions studied within recent Cochrane Reviews are not supported by high quality evidence, and harms are under-reported.