Abstract: Background: Although there is increasing evidence of the efficacy of psychedelic-assisted psychotherapy (PAP) for depressive and anxiety disorders, less is known regarding how different methodological aspects affect outcomes. This study investigated the moderating impact of one or multiple dosing sessions and type of adjunctive psychotherapy on the efficacy of PAP in depression and anxiety populations. Methods: This systematic review and meta-analysis aimed to include all published clinical trials of PAP investigating a primary outcome of symptoms of anxiety disorders or depressive disorders. Adjunctive psychotherapy, operationalized as a dichotomous variable either derived from evidence-based psychotherapy (EBP) or not, and number of dosing sessions were included in the analysis as moderators. Results: Twenty-three studies met inclusion criteria. Results indicated large to very large between and withingroup effects of PAP at post-treatment and follow-up, Hedge's g = 0.98-1.83. We found evidence of a moderating effect of the number of dosing sessions in the between-group analysis favoring two sessions over one. This finding was not replicated in the within-group analysis. Limitations: Only three studies included EBP-derived psychotherapy protocols which limited the possibility of investigating this moderator. Lack of blinded assessment of outcomes and non-adherence to the intention-to-treat principle affected risk of bias negatively. Conclusions: There was some evidence of a moderating effect of two versus one psychedelic dosing session, however this finding needs to be replicated using experimental designs since it was not robust across analyses. The field can be improved by more thorough reporting of psychotherapy protocols to facilitate comparison between studies.
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