Biological psychiatry. Cognitive neuroscience and neuroimagingFeb 22, 2024

How Expectations Influence Results in Psychedelic Studies

Psychedelic Science Weekly Brief ↗PubMed ↗DOI ↗

Updated Apr 2, 2026

Abstract

Expectancy effects may influence treatment outcomes in psychedelic therapy.

Participant expectations can significantly affect the results of psychedelic trials.
As doses of psychedelic compounds increase, maintaining a double-blind design becomes more challenging.
Expectancy effects have been observed in both micro- and macrodose psychedelic trials.
Expectancy could act as a separate physiological process that interacts with the drug's effects.
Managing expectancy effects may enhance the rigor and generalizability of future studies.

Simplified

Clinical trials of psychedelic compounds like psilocybin, lysergic acid diethylamide (LSD), and N,N-dimethyltrptamine (DMT) have forced a reconsideration of how nondrug factors, such as participant expectations, are measured and controlled in mental health research. As doses of these profoundly psychoactive substances increase, so does the difficulty in concealing the treatment condition in the classic double-blind, placebo-controlled trial design. As widespread public enthusiasm for the promise of psychedelic therapy grows, so do questions regarding whether and how much trial results are biased by positive expectancy. First, we review the key concepts related to expectancy and its measurement. Then, we review expectancy effects that have been reported in both micro- and macrodose psychedelic trials from the modern era. Finally, we consider expectancy as a discrete physiological process that can be independent of, or even interact with, the drug effect. Expectancy effects can be harnessed to improve treatment outcomes and can also be actively managed in controlled studies to enhance the rigor and generalizability of future psychedelic trials.

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How Expectations Influence Results in Psychedelic Studies

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