Open-label placebos: A systematic review and meta-analysis of experimental studies with non-clinical samples

Background: The use of open-label placebos (OLPs) has been shown to be effective in clinical trials. We conducted a systematic review and meta-analysis to examine whether OLPs are effective in experimental studies with non-clinical populations. Methods: We searched five electronic databases on April 15, 2021. We conducted separate analyses for self-reported and objective outcomes and examined whether the level of suggestiveness of the instructions influenced the effectiveness of OLPs. Results: Of 3,573 identified records, 20 studies comprising 1,201 participants were included. We found a significant effect of OLPs for self-reported outcomes (SMD=0.43; 95% CI=0.28, 0.58; I 2 = 7.2%) but not for objective outcomes (SMD=-0.02; 95% CI=-0.25, 0.21; I 2 = 43.6%). The level of suggestiveness of the instructions influenced the effectiveness of OLPs for objective outcomes ( p =.02), but not for self-reported outcomes. Discussion: OLPs appear to be effective when examined in experimental studies. However, the small number of studies highlights the need for further research.