Risk of bias in randomized clinical trials on psychedelic medicine: A systematic review

Background: The classical psychedelics, psilocybin, peyote, ayahuasca/ N,N-dimethyltryptamine, and lysergic acid diethylamide are considered promising new treatments for psychiatric illnesses, such as depression, anxiety, addiction, and obsessive-compulsive disorders. However, their profound and characteristic subjective effects raise concern for distinctive biases in randomized clinical trials.

Methods: We performed a systematic literature search to identify all clinical trials on classical psychedelics with patient populations to examine descriptive data and determine the risk of bias. Two independent reviewers searched three databases (PubMed, Embase, and APA PsycNet) and extracted information on study design, study population, use of active or inactive placebo, dropouts, evaluation of blinding of intervention, and reporting of expectancy and therapeutic alliance.

Results: We included 10 papers reporting on 10 unique trials. The trials generally included populations that were predominantly white and highly educated. The trials had small samples and considerable dropout. Blinding was either unsuccessful or not reported regardless of type of placebo. Few trials published protocols, statistical analysis plans (SAPs), and outcomes relating to psychotherapy fidelity. All trials but one were rated as high risk of bias.

Conclusion: Successful blinding of intervention is a significant challenge in this field. To better accommodate this, we suggest that future trials use a parallel-group design and utilize an active placebo on a psychedelic-naïve population. Future trials should publish trial protocol and SAPs, use clinician-rated outcomes accessed by a blinded rater, evaluate blinding of intervention, and consider measuring expectancy and therapeutic fidelity.