Psychedelics and Mental Health Series - 2/3 - Psychedelics and depression: how the question of blinding changes everything

Psychedelics and Depression: What the Issue of Blinding Changes - 2/3

The previous episode set the stage: a family of molecules reintegrated into contemporary science, supported by a coherent neurobiological model and new biomarkers. It remained to examine what the clinical trials themselves say. This is where things get complicated.

Impressive Figures, Widely Repeated

For several years, the specialized and general press has been relaying results that seem spectacular. A frequently cited meta-analysis, covering nine placebo-controlled trials, reported an average effect size of greater than 1 for psilocybin, which significantly exceeds the typical values for antidepressants or psychotherapy alone. For MDMA in post-traumatic stress disorder, two phase 3 trials published in 2021 and 2023 showed much stronger symptom reductions in the treated group than in the placebo group, with effect sizes considered large.

If we stop there, the conclusion seems obvious: psychedelic medicine performs better than existing treatments. But a methodological question disrupts this reading.

The Problem of Functional Unblinding

In a placebo-controlled trial, blinding (the fact that neither the patient nor the evaluator knows who is receiving the active treatment) is supposed to neutralize effects linked to expectation, suggestion, or the trial atmosphere. This is one of the pillars of evidence-based medicine.

Yet psychedelics pose a particular problem: their subjective effects are so recognizable that approximately 95% of participants correctly guess their assigned group. Blinding exists on paper, but it is broken in practice within the first hour of the session. Methodologists call this functional unblinding.

The consequences of this unblinding are not neutral. The active group, aware of receiving the innovative treatment, benefits from an expectation effect that amplifies the reported improvements. On the other hand, the placebo group often realizes they received nothing and experiences a disappointment effect that can worsen scores. The measured difference between the two groups therefore includes, in addition to the pharmacological effect, a psychotherapeutic expectation gap that is difficult to quantify.

When This Bias is Neutralized

A meta-analysis published in JAMA Psychiatry in 2025 by Barnett, Szigeti, and colleagues attempted an interesting exercise: comparing psilocybin and traditional antidepressants under equivalent blinding conditions, which means placing open trials (where everyone knows their treatment) and imperfectly blinded trials side by side. The result is unexpected for anyone who stops at the usual headlines: under equalized blinding conditions, the difference in efficacy between psilocybin and antidepressants becomes minimal, on the order of a few tenths of a point on the Hamilton scale.

This does not mean that psychedelics are ineffective. It means that their apparent superiority over existing treatments could, to a significant extent, be an artifact related to the very structure of the trials.

What Real-World Data Says

A recent Swiss cohort, in a compassionate use framework, documented the progress of just over a hundred adults with treatment-resistant depression after a single dose of LSD or psilocybin. The authors report a substantial reduction in rumination and catastrophizing, with a safety profile deemed acceptable in this supervised setting. These data do not settle the debate on blinding, but they do confirm clinical feasibility outside randomized trials and the consistency of the observed effects in practice.

What This Episode Changes for the Clinician

The useful takeaway is neither unconditional enthusiasm nor radical skepticism. Two observations coexist. On one hand, the biological signature of psychedelics, their rapid action, and their effect on cortical plasticity are robust. On the other hand, the demonstration of a clear clinical superiority over current treatments remains fragile as long as the question of blinding is not better managed.

In practice, this calls for discussing these molecules as serious options for resistant disorders, without turning them into miracle solutions or promoting them beyond what the evidence warrants. Rigor on this point is the condition for this field to be taken seriously in the long term.

The next episode will address the question that extends this one: assuming these treatments earn their clinical credentials, under what conditions can they be deployed on a large scale?