Masking Influences: A Systematic Review of Placebo Control and Masking in Psychedelic Studies

Psychedelic-assisted therapy is becoming increasingly acknowledged as an effective therapeutic intervention for various psychiatric illnesses. However, the evaluation of masking success is rarely reported in trials. The objective of the present systematic review was to evaluate placebo-control and masking in studies exploring psychedelic-assisted therapy. Nine (k = 9) studies dating between January 2010 and March 2023 were retrieved using six databases, following strict inclusion and exclusion criteria. The results show that almost 78% of the studies had, at best, "poor" masking success. At the same time, 60% of active placebo and 75% of inactive placebo studies showed large effect sizes. In other words, masking influences, including benign unmasking, cannot be excluded. We therefore conclude that efficacy of psilocybin, Ayahuasca, or LSD is only one of the possible interpretations of large, positive changes in symptomatology for patients suffering from, for example, alcohol use disorder, anxiety with or without life-threatening disease, anxiety and/or depression with life-threatening cancer, treatment-resistant depression or major depressive disorder. We recommend care be taken to increase successful masking procedures and discuss alternative treatment designs to better control for potential masking influences.