Placebo-controlled study suggests that the benefits of psilocybin microdosing can be explained by expectancy effects

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Anecdotal evidence suggests that microdosing with psilocybin mushrooms offers mental health benefits. But a recent experimental study suggests that these purported benefits may be driven by users’ expectations. Findings from the placebo-controlled experiment were published in the journal Translational Psychiatry.

Microdosing, the practice of consuming low, sub-hallucinogenic doses of psychedelics, appears to be growing in popularity among the public. People who engage in microdosing often do so for the potential mental health benefits, hoping the practice will improve their mood, concentration, cognitive function, and even creativity. However, there is currently little experimental research on the effects of microdosing with psychedelics.

In an experimental study, Federico Cavanna and colleagues tested the effects of microdosing with psilocybin — a compound found in psychoactive mushrooms that is commonly used for microdosing. Importantly, the study authors controlled for participant and experimenter expectancy effects, which is when the expectations of participants and researchers influence the results of the study.

The researchers recruited 34 participants (11 women) who were already planning to start a microdosing protocol. The experiment involved two weeks of measurements. During one week, participants were given two doses of 0.5 g of dried psilocybin mushrooms. During another week, participants were given two doses of placebo of the same weight and preparation. The experiment was double-blind, which meant that the conditions were organized by a third party, and neither the experimenters nor the participants knew if subjects were given a placebo or psilocybin capsule.

Participants completed a self-report questionnaire where they reported the acute effects they experienced with the doses (e.g., “My sense of space and size was distorted”) and completed psychological measures including anxiety, positive and negative affect, well-being, and stress. They also completed several tasks assessing creativity, perception, and cognition, and their brain activity was measured with electroencephalography (EEG). Finally, participants reported their expectations for how their mental state might change in various areas (e.g., positive emotion, anxiety).

The results revealed significant effects on the self-report questionnaire, where participants taking psilocybin reported higher acute effects compared to those taking placebo. However, these results were only significant among subjects who had correctly identified which condition they were in — in other words, subjects who correctly identified whether or not they were taking psilocybin. This suggests that the observed subjective effects were driven by participants’ expectations.

While the EEG results revealed altered EEG rhythms, the study failed to reveal any positive impact of psilocybin on subjects’ creativity, cognition, or self-reported mental well-being. By contrast, a trend in the results suggested that the psilocybin may have hindered performance on certain cognitive tasks. The authors note that this trend is in line with past evidence suggesting that stronger doses of serotonergic hallucinogens can be detrimental to cognitive functioning, for example, by impairing attention and decision-making.

Overall, the results did not support previous evidence that microdosing improves well-being, creativity, or cognitive function. However, there were several limitations to the study which may have impacted the findings.

For one, the experiment involved a short-term microdosing schedule of two doses per week. Future studies should test whether microdosing over an extended period may have a stronger effect on mental health. Additionally, the sample was comprised of healthy participants, and it could be that microdosing with psilocybin only produces positive benefits among patients suffering from mental health issues. The authors say additional research is needed to determine whether microdosing truly offers mental health benefits, and to gain a clearer picture of its safety.

The study, “Microdosing with psilocybin mushrooms: a double-blind placebo-controlled study”, was authored by Federico Cavanna, Stephanie Muller, Laura Alethia de la Fuente, Federico Zamberlan, Matías Palmucci, Lucie Janeckova, Martin Kuchar, Carla Pallavicini, and Enzo Tagliazucchi.

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