Full title: Most healthcare interventions tested in Cochrane Reviews are not effective according to high quality evidence: a systematic review and meta-analysis https://www.sciencedirect.com/science/article/abs/pii/S0895435622001007 Highlights • In this large sample of 1567 interventions studied within Cochrane reviews, effects of most interventions (94%) interventions were not supported by high quality evidence. • Potential harms of healthcare interventions were measured more rarely than benefits. • Patients, doctors, and policy makers should consider the lack of high-quality evidence supporting the benefits and harms of many interventions in their decision-making. 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.