Meta-analyses frequently include old trials that are associated with a larger intervention effect: a meta-epidemiological study

•Trials published before 2000 represented one fourth of all trials included in meta-analyses and trials published before 1990 almost 10%. The oldest trial was published in 1951.•Intervention effects were, on average, significantly larger for older than recent trials.•Results were consistent in sensitivity analyses adjusted on risk of bias and sample size.•It is generally recommended to include all trials within a meta-analysis whatever their publication date but this may have an influence on external validity and intervention effect.•With a biomedical literature that is increasing exponentially, we wonder whether it is reasonable to consider the results of old trials sometimes conducted more than 50 years ago. To assess whether meta-analyses include older randomized controlled trials (RCTs) and whether intervention effect differ between older and recent RCTs. In this meta-epidemiological study of 295 meta-analyses (2940 RCTs) published in 2017–2018, we evaluated the difference in intervention effects between older (i.e., published before 2000) and recent RCTs. We also compared effects by quarters of publication year within each meta-analysis (from quarter 1 including the 25% oldest trials to quarter 4 including the 25% most recent trials). A ratio of odds ratio (ROR)