Blinding and Expectancy Confounds in Psychedelic Randomised Controlled Trials

There is increasing interest in the potential for psychedelic drugs such as psilocybin, LSD and ketamine to treat a number of mental health disorders. To gain evidence for the therapeutic effectiveness of psychedelics, a number of randomised controlled trials (RCTs) have been conducted using the traditional RCT framework and these trials have generally shown promising results, with large effect sizes reported. However, in this paper we argue that estimation of treatment effect sizes in psychedelic clinical trials are likely over-estimated due to de-blinding of participants and high levels of response expectancy generated by RCT trial contingencies. The degree of over-estimation is at present difficult to estimate. We conduct systematic reviews of psychedelic RCTs and show that currently reported RCTs have failed to measure and report expectancy and malicious de-blinding. In order to overcome these confounds we argue that RCTs should routinely measure de-blinding and expectancy and that careful attention should be paid to the clinical trial design used and the instructions given to participants to allow these confounds to be estimated and removed from effect size estimates. We urge caution in interpreting effect size estimates from extant psychedelic RCTs.