Masking influences: A systematic review of placebo control and blinding in psychedelic studies

Abstract

Abstract Psychedelic-assisted therapy is becoming increasingly acknowledged as an effective therapeutic intervention for various psychiatric illnesses. However, the evaluation of blinding success is rarely reported in trials. The objective of the present systematic review was to evaluate placebo-control and blinding in psychedelic-assisted therapy. Nine studies dating between January 2010 until March 2023 were retrieved following strict inclusion and exclusion criteria. The results show that all studies had either poor or uncertain blinding success, and all studies that used an active placebo control had poor blinding success. In other words, we cannot exclude the possibility of masking influences. For instance, this may lead to those receiving placebo having less expectations of treatment, resulting in a smaller placebo effect, and in turn the appearance of a more effective treatment than is the case. We recommend care be taken to increase successful blinding procedures, and we discuss alternative treatment designs to better control for potential masking influences.