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Blinding and expectancy confounds in psychedelic randomized controlled trials
Abstract
Introduction: There is increasing interest in the potential for psychedelic drugs such as psilocybin, LSD and ketamine to treat several mental health disorders, with a growing number of randomized controlled trials (RCTs) being conducted to investigate the therapeutic effectiveness of psychedelics.Areas covered: We review previous literature on expectancy effects and blinding in the context of psychedelic RCTs – literature which strongly suggest that psychedelic RCTs might be confounded by de-blinding and expectancy. We conduct systematic reviews of psychedelic RCTs using Medline, PsychInfo and EMBASE (Jan 1990 – Nov 2020) and show that currently reported psychedelic RCTs have generally not reported pre-trial expectancy, nor the success of blinding procedures.Expert opinion: While psychedelic RCTs have generally shown promising results, with large effect sizes reported, we argue that treatment effect sizes in psychedelic RCTs are likely over-estimated due to de-blinding of participants and high levels of response expectancy. We suggest that psychedelic RCTs should routinely measure de-blinding and expectancy. Careful attention should be paid to clinical trial design and the instructions given to participants to allow these confounds to be reduced, estimated and removed from effect size estimates. We urge caution in interpreting effect size estimates from extant psychedelic RCTs.
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“systematic unblinding has been raised as a major threat to the validity of extant findings from ‘double blind’ PAT trials due to high expectancy, presumably resulting in large placebo and nocebo effects in the experimental group and the control group, respectively [32–34].”
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“leading to a general call for more effective active placebo conditions and a systematic reporting of blinding efficacy [32–34].”
In the citing paper
“proposing an active placebo condition aimed at maintaining blinding, while systematically assessing and reporting blinding efficacy in participants and therapists.”
In the citing paper
“blinding efficacy (participants, therapists) and expectancy (participants) will be assessed and reported, as recommended in the literature [32–34].”
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“…treatment and AEs; effectively blinding study participants to their treatment condition is notably hard in psychedelic drug studies (Aday et al., 2022; Muthukumaraswamy et al., 2021), particularly given the high percentages of patients who have had previous experience with a psychedelic.”
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Metadata
- Authors
- Muthukumaraswamy, S. D., Forsyth, A., & Lumley, T.
- Affiliations
- University of Auckland (NZ)
- Corresponding author
- Muthukumaraswamy, Suresh D.
- Venue
- Expert Review of Clinical Pharmacology
- Publisher
- Informa UK Limited
- Published
- 2021-08-26
- Volume
- 14
- Issue
- 9
- Pages
- 1133-1152
- ISSN
- 1751-2433 (ISSN-L)
- 1751-2441
- OpenAlex
- W3164618783
- Cited by
- 324
- References
- 128
- Citations by year
- 2026: 57 · 2025: 75 · 2024: 81 · 2023: 61 · 2022: 42 · 2021: 8
- FWCI
- 6.38
- Type
- journal article
- Language
- en
- Open access
- closed
- Locations
- Expert Review of Clinical Pharmacology (publishedVersion)
- PubMed (publishedVersion)
- Funders
- Health Research Council of New Zealand
- Topics
- Social Sciences › Psychology › Clinical Psychology › Psychedelics and Drug Studies
- Life Sciences › Neuroscience › Cognitive Neuroscience › Pain Management and Placebo Effect
- Physical Sciences › Mathematics › Statistics and Probability › Statistical Methods in Clinical Trials
- Keywords
- psychedelics, blinding, response expectancy, confounding, psilocybin, LSD, ketamine, effect size overestimation, trial design, mental illness
- MeSH terms
- Mental Disorders, Hallucinogens, Humans, Ketamine, Lysergic Acid Diethylamide, Psilocybin, Research Design, Confounding Factors, Epidemiologic, Randomized Controlled Trials as Topic
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