The invitation e-mail was sent to the trialists in 24 countries in total, with China and USA on the top with 29 trials followed by Germany, Korea, and UK. Among 100 studies that replied to our invitation letter (62.5% of response rate), 63 studies provided PILs. In addition, one author generously provided two extra PILs on top of the requested documents and final 65 PILs from 15 different countries were collected for analysis (Fig. 1; Additional file 1: Appendix 1).Fig. 1Flow diagram for selection of studies. aAmong included studies, only those providing extractable data for calculating blinding index were included here. bStudies that clearly indicated and reported specific data of primary outcome were eligible for meta-analysis. RCT, randomized controlled trial