Summary
Background. After half a century of prohibition, psychedelics have returned to the forefront of psychiatric research as potential treatments for depression, post-traumatic stress disorder, and addiction. Enthusiasm, however, has often outpaced the evidence.
Objectives. To critically examine the state of evidence regarding psychedelic and related compounds in psychiatry, distinguishing genuine clinical signals from the methodological and regulatory vulnerabilities that have tempered expectations over the past two years.
Methods. A critical review of international and Italian literature, firmly anchored to verified primary sources.
Results. Psilocybin demonstrates a promising yet non-definitive antidepressant signal: the most rigorous head-to-head comparison failed to establish its superiority over escitalopram regarding the primary outcome. MDMA-assisted therapy for PTSD, once deemed imminent, was rejected by the FDA in 2024; the sole agent in this domain to enter clinical practice remains esketamine. A profound methodological challenge—the impossibility of true blinding—permeates and undermines the entire field.
Conclusions. The promise is genuine and warrants rigorous research; however, the field’s credibility hinges on its ability to resolve the blinding issue, disentangle the pharmacological effect from psychotherapy and expectancy, and resist prevailing commercial and ideological pressures. It is not a panacea: it is a serious hypothesis that still demands empirical proof.
Keywords: psychedelics, psilocybin, MDMA, ketamine, esketamine, treatment-resistant depression, PTSD, functional unblinding, evidence-based medicine.
Abstract
Background. After half a century of prohibition, psychedelics have returned to the centre of psychiatric research as candidate treatments for depression, post-traumatic stress disorder and addictions. Enthusiasm, however, has often outrun the evidence.
Objectives. To examine critically the state of the evidence on psychedelic and related compounds in psychiatry, distinguishing the genuine clinical signal from the methodological and regulatory weaknesses that, over the past two years, have tempered expectations.
Methods. Critical survey of the international and Italian literature, anchored to verified primary sources.
Results. Psilocybin shows a promising but not decisive antidepressant signal: the most rigorous head-to-head comparison did not find it superior to escitalopram on the primary outcome. MDMA-assisted therapy for PTSD, widely expected to be approved, was rejected by the FDA in 2024. The only agent in this area to reach the clinic is esketamine. A methodological problem — the impossibility of true blinding — runs through and weakens the whole field.
Conclusions. The promise is real and deserves rigorous research; but the field’s credibility depends on solving the blinding problem, on disentangling drug from psychotherapy and expectancy, and on resisting commercial and ideological pressure. It is not a panacea: it is a serious hypothesis, still to be proven.
Keywords: psychedelics, psilocybin, MDMA, ketamine, esketamine, treatment-resistant depression, PTSD, functional unblinding, evidence-based medicine.
- Introduction: The Return, and the Reckoning
During the 1950s and 1960s, LSD and psilocybin were the subject of over a thousand scientific publications, with experimental applications in depression, alcoholism, and the psychological distress of terminally ill patients. The “War on Drugs” abruptly ended that era: in 1970, psychedelics were classified in the United States as Schedule I substances, deemed to possess no recognized medical use and a high potential for abuse, effectively extinguishing clinical research for a generation. Subsequently, beginning in the early 2000s, this research re-emerged—heralding what Michael Pollan (2018) aptly described as the return of psychedelia in a white coat.
The dominant narrative of this “renaissance” has been overwhelmingly triumphant: molecules capable of curing in a single session what traditional antidepressants fail to touch over years, a reopened consciousness, and a fundamentally refounded psychiatry. This review proceeds from a different, and hopefully more constructive, premise: that the history of the past two years has been, above all, a reckoning.
In 2024, the United States Food and Drug Administration (FDA) rejected MDMA-assisted therapy for post-traumatic stress disorder, an approval that many had considered a foregone conclusion; furthermore, the most rigorous head-to-head comparison between psilocybin and a traditional antidepressant failed to demonstrate the anticipated superiority on its primary outcome; and a fundamental methodological flaw—the impossibility of conducting genuinely blinded trials—has continually undermined every finding. The objective here is to reconcile two truths that the prevailing clamor tends to divide: the clinical signal is real and must be taken seriously; yet the chasm between promise and proof remains vast, and if anything, it has widened over the past two years.
- The Overarching Dilemma: The Impossible Blind
It is prudent to begin with the issue that, more than any other, colors the interpretation of all subsequent evidence, for it represents the Achilles’ heel of the entire field. Evidence-based medicine is founded upon the randomized, double-blind trial: neither the patient nor the investigator should know who has received the active drug and who has received the placebo. When dealing with a substance that radically alters perception, consciousness, and emotionality for several hours, this requirement becomes practically unachievable: virtually all participants immediately deduce which study arm they have been assigned to. This phenomenon is known as functional unblinding.
The consequences are profound. If a patient knows they have received a highly anticipated, “miraculous” substance—one they perhaps actively sought out and are deeply enthusiastic about—the expectation itself can generate a substantial portion of the clinical improvement, thereby inflating the estimate of the drug’s specific effect (Muthukumaraswamy, Forsyth & Lumley, 2021). This is not a mere statistical technicality: it is the primary reason why many spectacular results must be interpreted with extreme caution, and why the most rigorous comparative studies tend to significantly temper them. Anyone who discusses psychedelics in psychiatry without acknowledging this fundamental issue is evaluating the data through a distorted lens.
- Psilocybin and Depression: The Signal and Its Limitations
Psilocybin, an agonist of the 5-HT2A serotonergic receptors, is the compound upon which the greatest expectations converge. The seminal studies of 2016, conducted on oncology patients suffering from disease-related anxiety and depression, demonstrated broad and enduring symptom reductions following a single administration in a highly controlled setting (Griffiths et al., 2016; Ross et al., 2016). A subsequent randomized trial documented its efficacy in major depressive disorder (Davis et al., 2021). Upon this foundation, the image of psilocybin as a next-generation antidepressant was forged.
However, two more recent studies necessitate a much more sober interpretation. The most rigorous comparative study conducted to date—a double-blind trial pitting psilocybin against escitalopram, a selective serotonin reuptake inhibitor—failed to demonstrate any statistically significant difference between the two treatments regarding the primary outcome (the change in depressive symptom score at six weeks); while several secondary outcomes trended in favor of psilocybin, these were not corrected for multiple comparisons and therefore do not permit definitive conclusions (Carhart-Harris et al., 2021). This is a critical finding that the popular narrative frequently omits, yet it must be placed front and center.
The largest randomized trial on psilocybin, for its part, demonstrated that a single 25 mg dose reduced symptoms of treatment-resistant depression more effectively than a negligible control dose; nevertheless, this trial was designed and funded by the very company developing the compound, it compared different doses of the same drug rather than utilizing a true placebo, and its long-term effects remain entirely uncertain (Goodwin et al., 2022). In summary, the clinical signal is present; however, it has not yet been validated by the caliber of evidence that psychiatry rightfully demands of its pharmacopeia.
- MDMA and Post-Traumatic Stress Disorder: Anatomy of a Rejection
MDMA acts as an entactogen: it facilitates emotional processing and reduces fear and avoidance, which is why it was proposed not as a standalone medication, but rather as a catalyst for structured psychotherapy aimed at treating post-traumatic stress disorder. A Phase III clinical trial had reported remarkable results, with remission rates significantly exceeding those achieved by conventional psychotherapies (Mitchell et al., 2021). Riding the wave of these data, many considered regulatory approval a certainty, anticipating the arrival of the first novel therapy for PTSD in over two decades.
This did not come to pass, making it the most instructive episode of the last five years. On June 4, 2024, the FDA advisory committee voted by a wide margin that the evidence of efficacy was insufficient and that the potential benefits did not outweigh the risks. Subsequently, on August 9, 2024, the FDA formally rejected the application, requesting that the sponsor conduct an additional Phase III trial.
The rationale aligns exactly with the aforementioned concerns, and then some: functional unblinding and the ensuing expectancy bias, which were further exacerbated by the fact that many participants had prior experience with MDMA; inadequately characterized cardiovascular risks and abuse potential; and, most egregiously, allegations of sexual misconduct by therapists during one of the trials. In the aftermath of this decision, a scientific journal retracted three articles that had published data from that research program. This marked the first instance of the FDA evaluating a Schedule I psychedelic for medical use, and the message delivered was unequivocal: enthusiasm, and even the political pressure exerted by veterans and legislators, cannot act as a substitute for scientific proof (Roseman, 2025).
- Ketamine and Esketamine: The Exception That Reached the Clinic
While classic psychedelics remain largely confined to the realm of research, a related compound is already in clinical use, and understanding why is highly instructive. Ketamine, an NMDA glutamate receptor antagonist and a longstanding dissociative anesthetic, produces a rapid antidepressant effect—often within hours—in patients suffering from treatment-resistant depression: this was demonstrated by a now-seminal study funded by the US National Institutes of Mental Health, showing a remarkably large effect size at twenty-four hours (Zarate et al., 2006). From this foundation arose the 2019 FDA approval of intranasal esketamine (Spravato) for treatment-resistant depression: it stands as the sole pharmacological agent in this entire domain to have successfully transitioned into routine clinical practice.
Two points are worth noting. Firstly, ketamine is not a classic psychedelic: it does not act upon serotonergic receptors, and its very classification within “psychedelia” remains a subject of debate. Secondly, real-world clinical experience has significantly tempered its initial aura: a large-scale Italian multicenter study—the REAL-ESK study—confirmed the efficacy and reasonable safety profile of esketamine in routine practice, but concurrently highlighted the absolute necessity of rigorous patient selection, meticulous monitoring, and the careful management of dissociative side effects (Martinotti et al., 2022). Ketamine serves as a contrasting lesson on what is required for a compound in this field to cross the regulatory threshold: years of successful replication, a well-characterized mechanism of action, and adherence to standard regulatory pathways—not exceptionalism.
- Neurobiology and Explanatory Models
On a mechanistic level, classic psychedelics act as agonists of cortical 5-HT2A receptors, thereby altering perception, emotional processing, and the sense of self (Nichols, 2016; Vollenweider & Kometer, 2010). The most widely discussed neurofunctional hypothesis is the REBUS (Relaxed Beliefs Under Psychedelics) model proposed by Carhart-Harris and Friston (2019): by temporarily dampening the activity of the Default Mode Network—the neural network associated with the narrative self and rumination—and thereby loosening the rigidity of deeply entrenched beliefs, psychedelics theoretically render the brain more plastic and receptive to the restructuring of dysfunctional cognitive schemas.
This is an elegant and clinically evocative model, yet it must be handled with the same degree of caution applied to the clinical data. Describing a neural correlate of a mental state is by no means equivalent to demonstrating a therapeutic effect, and the peril of falling into neurochemical reductionism—attempting to explain clinical recovery merely through a magnetic resonance image—is remarkably tangible. The mechanism simply renders the efficacy biologically plausible; it does not substitute for proof of that efficacy.
- Risks, Subjective Experience, and the Cultural Question
The depiction of these substances as intrinsically innocuous because they are “natural” or “spiritual” is dangerously misleading. Psychedelics can precipitate severe acute adverse reactions—often termed “challenging experiences”—and, in vulnerable individuals, unmask underlying psychotic disorders: a recent Italian review aptly characterized the relationship between psychedelics and schizophrenia as a “double-edged sword” (Sapienza et al., 2025). Furthermore, there exists the phenomenon of Hallucinogen Persisting Perception Disorder (HPPD), which has been studied within the Italian context under the broader framework of new psychoactive substances (Orsolini et al., 2017).
Moreover, there is a profound ethical dilemma, made dramatically concrete by the MDMA trials: a patient under the influence of these substances enters a state of extreme vulnerability and suggestibility, exposing them to significant risks of abuse and undue influence by the therapist.
On the flip side, the literature clearly documents that efficacy, where it exists, does not reside solely in the molecule itself, but rather in the complex interplay of preparation, context (the ‘set and setting’), the subjective experience, and subsequent integration: the substance is merely a component of a broader process, not the process in its entirety. This raises a crucial, currently unresolved question: what proportion of the observed effect is truly pharmacological, and what proportion is psychotherapeutic and cultural? This is the precise question that the FDA committee posed regarding MDMA, and it is the fundamental reason why the “drug plus psychotherapy” paradigm represents such a slippery regulatory construct.
- Regulation, the Market, and the Italian Framework
The regulatory landscape is rapidly evolving and is characterized by a deep underlying tension. On one side lies the medical pathway, defined by controlled clinical trials and the potential for standard pharmaceutical approval; on the other lies the path of selective legalization for facilitated use, already embarked upon by several US states (such as Oregon and Colorado), fueled by the momentum of a market that has already attracted massive capital investments. Between these two paradigms emerges the perilous space of commercial medicalization, which risks outpacing the empirical evidence—a danger that the most astute scientific literature consistently warns against (Nutt et al., 2010; Reiff et al., 2020).
In Italy, the field presents itself in a highly instructive manner. Classic psychedelics remain strictly confined to research, and the national scientific community has adopted a lucid, unembellished stance: a recent Italian-authored review retraced the case of psilocybin precisely through the lens of this tension between scientific excitement, evidence of efficacy, and real-world challenges (Scala et al., 2024). The sole clinical reality, in Italy as elsewhere, remains esketamine, whose utilization has been well-documented by the previously cited Italian multicenter study (Martinotti et al., 2022). This serves as yet another confirmation of a simple rule: what enters clinical practice is that which has successfully navigated the rigors of standard empirical testing, not that which has generated the highest expectations.
- Limitations and Conclusions
This review represents a selective critical reconnaissance guided by a specific thesis—the distance between promise and evidence—and does not claim to be exhaustive: DMT and ayahuasca, the treatment of addiction, and the novel non-hallucinogenic analogues would each merit a dedicated discussion. However, the overarching position that emerges is clear, and it is best stated without pretense.
Psychedelics and their related compounds are neither the panacea heralded by enthusiasts nor the elaborate fraud denounced by the most superficial skeptics. They constitute a serious therapeutic hypothesis, possessing a genuine clinical signal primarily in the context of depression and post-traumatic stress disorder, with esketamine serving as the sole established clinical reality. Yet, they also represent a field where enthusiasm has sprinted far ahead of the evidence, ultimately stumbling: the FDA’s rejection of MDMA in 2024, and the statistical tie between psilocybin and a standard antidepressant in 2021, serve as two inescapable reminders that no honest scientific review can circumvent.
The future credibility of this field will be determined on three fronts: successfully resolving, or at least accurately measuring, the confounding effects of unblinding and expectancy; honestly disentangling the specific pharmacological contribution from that of psychotherapy and cultural setting; and firmly resisting the dual pressures—both commercial and ideological—that seek to treat the conclusion as already written. Until that standard is met, the appropriate term is not revolution, but rather prudence—the exact prudence owed to any profound medical promise that asks to be believed before it has been definitively proven.
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