Impact of psilocybin on cognitive function: A systematic review.

Psilocybin is a classic psychedelic with demonstrated preliminary clinical efficacy in a range of psychiatric disorders. Evaluating the impact of psilocybin on cognitive function is essential to unravel its potential benefits and risks. In this systematic review, we assessed psilocybin's effect on cognitive function through a comprehensive search of electronic databases from inception to January 2024, identifying 20 articles involving 2,959 participants. While 85% of studies were conducted in healthy volunteers, most of these studies (85%) used macrodoses, ranging from 45 µg/kg to 30 mg/70 kg. Various cognitive aspects were evaluated and yielded mixed results. Global cognitive function, and processing speed remained mostly unchanged in healthy individuals; However, a limited number of studies reported improvements in certain areas such as sustained attention, working memory, and executive function especially in patients with treatment-resistant depression (TRD). Emotional processing was positively modified, particularly in TRD patients. Psilocybin was observed to enhance emotional empathy without significantly altering cognitive empathy and social cognition. Cognitive flexibility and creative cognition were noted to initially decline but could potentially improve over time. Additionally, with respect to learning and memory skills, psilocybin showed promise in improving specific memory types such as semantic associations and associative learning, while its effects on episodic and verbal memory have been less pronounced compared to other cognitive enhancers. The observed mixed findings underscore the complexity of psilocybin's cognitive influence. Further research is essential to provide a clearer understanding of psilocybin's impact on cognitive domains and to guide the development of safe and effective interventions.

Chemistry/structural biology of psychedelic drugs and their receptor(s).

This brief review highlights some of the structure-activity relationships of classic serotonergic psychedelics. In particular, we discuss structural features of three chemotypes: phenethylamines, ergolines and certain tryptamines, which possess psychedelic activity in humans. Where they are known, we point out the underlying molecular mechanisms utilized by each of the three chemotypes of psychedelic molecules. With a focus on the 5-HT2A receptor subtype, a G-protein coupled receptor known to be the primary target of psychedelics, we refer to several X-ray and cryoEM structures, with a variety of ligands bound, to illustrate the underlying atomistic basis for some of the known pharmacological observations of psychedelic drug actions.

Beyond the numbers: reimagining healing with psychedelics for eating disorders.

Psychedelic medicine is currently being evaluated for numerous mental health indications, and there is significant interest in applying these models of care to eating disorders (EDs) given the limited efficacy of available treatment models, especially for those living with anorexia nervosa. Preliminary findings across a number of studies suggest promise. In this commentary, researchers with experience in psychedelics and EDs present a rationale and considerations for the application of psychedelic medicine, including psychedelic-assisted therapy (PAT) for EDs. These contributions are informed by those with lived experience as well as the authors' experiences in the field. By addressing underlying psychological and transpersonal factors and improving treatment engagement, psychedelic medicine, though not without risks, may offer a valuable adjunct to existing treatments, enhancing overall outcomes for some living with an ED. This commentary also aims to provide a multi-dimensional perspective to inform the field, including with respect to the etiology of these illnesses, as psychedelic medicine becomes more accessible in naturalistic, research and clinical settings.

Psilocybin for major depressive disorder: a systematic review of randomized controlled studies.

Objectives: The purpose of this systematic review of randomized controlled trials (RCTs) was to evaluate the effectiveness, safety, and tolerability of psilocybin in adult patients with major depressive disorder (MDD). Methods: A systematic search (up to September 14, 2023) was conducted for RCTs that examined the efficacy, safety, and tolerability of psilocybin in physically healthy adult patients with MDD. Three independent researchers extracted data from publications where the primary outcome was a change in depressive symptoms, and key secondary outcomes were changes in anxiety symptoms and suicidal ideation, discontinuation rates for any reason, and adverse drug reactions (ADRs). Results: Five RCTs with 472 adult patients with MDD on psilocybin (n = 274) and controls (n = 198) were included. Two of the five RCTs (40%) reported mixed results, while the other three (60%) found that psilocybin had a beneficial effect on MDD treatment. Four RCTs (80%) assessing the anxiolytic effects of psilocybin for treating MDD found that psilocybin was significantly more effective than the control group in improving anxiety symptoms. Psilocybin was more effective than the control group in improving suicidal ideation in one out of five RCTs. Discontinuation rates were similar for any reason between the psilocybin group (2-13%) and the control group (4-21%) (P > 0.05). Four RCTs (80%) reported ADRs in detail. The most common ADR in both groups was headache. Conclusion: Psilocybin was effective in improving depressive symptoms in over half of the included studies and reduced anxiety symptoms in patients with MDD. The long-term efficacy and safety of psilocybin for MDD treatment needs to be further investigated in large RCTs.

What do health professionals think about implementing psilocybin-assisted therapy in palliative care for existential distress? A World Café qualitative study.

OBJECTIVES: Promising studies show that psilocybin-assisted therapy relieves existential distress in patients with serious illnesses, a difficult condition to treat with current treatment options. There is growing interest in this therapy in palliative care. Canada recently amended its laws to allow physicians to request psilocybin for end-of-life distress. However, barriers to access remain. Since implementing psilocybin-assisted therapy within palliative care depends on the attitudes of healthcare providers willing to recommend it, they should be actively engaged in the broader discussion about this treatment option. We aimed (1) to identify issues and concerns regarding the acceptability of this therapy among palliative care professionals and to discuss ways of remedying them and (2) to identify factors that may facilitate access. METHODS: A qualitative study design and World Café methodology were adopted to collect data. The event was held on April 24, 2023, with 16 palliative care professionals. The data was analyzed following an inductive approach. RESULTS: Although participants were interested in psilocybin-assisted therapy, several concerns and needs were identified. Educational and certified training needs, medical legalization of psilocybin, more research, refinement of therapy protocols, reflections on the type of professionals dispensing the therapy, the treatment venue, and eligibility criteria for treatment were discussed. SIGNIFICANCE OF RESULTS: Palliative care professionals consider psilocybin-assisted therapy a treatment of interest, but it generates several concerns. According to our results, the acceptability of the therapy and the expansion of its access seem interrelated. The development of guidelines will be essential to encourage wider therapy deployment.

Psilocybin and the glutamatergic pathway: implications for the treatment of neuropsychiatric diseases.

In recent decades, psilocybin has gained attention as a potential drug for several mental disorders. Clinical and preclinical studies have provided evidence that psilocybin can be used as a fast-acting antidepressant. However, the exact mechanisms of action of psilocybin have not been clearly defined. Data show that psilocybin as an agonist of 5-HT2A receptors located in cortical pyramidal cells exerted a significant effect on glutamate (GLU) extracellular levels in both the frontal cortex and hippocampus. Increased GLU release from pyramidal cells in the prefrontal cortex results in increased activity of γ-aminobutyric acid (GABA)ergic interneurons and, consequently, increased release of the GABA neurotransmitter. It seems that this mechanism appears to promote the antidepressant effects of psilocybin. By interacting with the glutamatergic pathway, psilocybin seems to participate also in the process of neuroplasticity. Therefore, the aim of this mini-review is to discuss the available literature data indicating the impact of psilocybin on glutamatergic neurotransmission and its therapeutic effects in the treatment of depression and other diseases of the nervous system.

Examining differences in the effects and contexts of naturalistic psilocybin use for White participants vs. participants of Color: A longitudinal online survey study.

BACKGROUND: Psilocybin (a psychoactive compound found in "magic mushrooms" or "shrooms") has been gaining increased attention in research and popular culture as a number of clinical and observational studies have demonstrated that it may have potential for improving mental wellbeing. Relatedly, there has been a substantial uptick in naturalistic (e.g., real-world, non-clinical) psilocybin use in the United States. While a number of longitudinal studies have demonstrated that naturalistic psilocybin use is linked to positive mental health outcomes on average, few studies have examined how the effects of psilocybin and contexts for psilocybin use may differ for White populations compared to Populations of Color. OBJECTIVE: To examine differences in health outcomes, subjective effects, and contexts of naturalistic psilocybin use in White participants compared to Participants of Color. METHODS: This study used data from a large, online longitudinal study of individuals who planned to engage in naturalistic psilocybin use (N = 2833). We used mixed-effects models to assess whether race/ethnicity (White vs. Participant of Color) moderated associations between time (Time 2 [initial assessment point for longitudinal measures], Time 5 [2-4 weeks post-psilocybin experience, and Time 6 [2-3 months post-experience]) and outcomes related to mental health (depression, anxiety, spiritual wellbeing, cognitive flexibility, emotion regulation [expressive suppression + cognitive reappraisal]). We also used exploratory chi-squared tests to examine differences in contexts for psilocybin use as well as differences in subjective effects related to the psilocybin experience. RESULTS: Race/ethnicity moderated the associations between time for predicting spiritual wellbeing (beta = -1.8; 95 % CI [-3.4, -0.17]; p < 0.05), cognitive flexibility (beta = -1.5 [-2.7, -0.26]; p < 0.05), and emotion regulation - expressive suppression (beta = 0.25 [0.06, 0.44]; p < 0.05) at Time 6 (but not Time 5). Additionally, Participants of Color reported minor differences in subjective effects and context for use compared to White participants (e.g., more likely to have set an intention prior to use, report time speeding up during the experience, etc.). We found reductions in anxiety and depression for both Participants of Color and White participants, and our moderation tests for these outcomes were not significant. CONCLUSION: Race/ethnicity impacts the associations between psilocybin use and various markers of mental wellbeing. Future longitudinal studies and experimental studies with larger samples of color can further elucidate the preliminary findings from this study.

Substrate recognition by the 4-hydroxytryptamine kinase PsiK in psilocybin biosynthesis.

Psilocybin, the natural hallucinogen from Psilocybe (magic) mushrooms, is a highly promising drug candidate for the treatment of depression and several other mental health conditions. Biosynthesis of psilocybin from the amino acid l-tryptophan involves four strictly sequential modifications. The third of these, ATP-dependent phosphorylation of the intermediate 4-hydroxytryptamine, is catalysed by PsiK. Here we present a crystallographic analysis and a structure-based mutagenesis study of this kinase, providing insight into its mode of substrate recognition. The results of our work will support future bioengineering efforts aimed at generating variants of psilocybin with enhanced therapeutic properties.

Expert recommendations for Germany's integration of psychedelic-assisted therapy.

As clinical trials for psychedelics move into phase III in the USA, Europe must address its lag in integrating professional education around psychedelic-assisted therapy (PAT) and supporting psychedelic drug research. This paper evaluates the necessary frameworks for implementing PAT in Germany, emphasizing the nation's potential leadership role within the European Union. With Australia having already approved MDMA and psilocybin for mental health indications, the Ukrainian government exploring MDMA treatment for war-related PTSD, and initial clinical trials involving MDMA and LSD with patients in Switzerland which restarted the restricted medical use of these substances around 2014, the medical authorization of psychedelics in these countries establishes precedent showcasing both the promise and challenges of researching and implementing PAT in nations where the substances were formally scheduled as illicit substances. Key challenges include establishing rigorous standards for practitioner training, accessibility, and defining regulatory oversight. This paper focuses on the development of robust infrastructure in Germany, which will support the roll out of PAT, and details ethical considerations, training protocols, and governmental roles in the formulation of treatment frameworks. This approach aims not only to guide Germany in adopting PAT but also to influence broader European policy, ensuring that patients receive ethically sound and proficient care. The findings suggest pathways for Europe to reclaim its historical lead in psychiatric and therapeutic innovation.

A Case Report of Psilocybin-induced Psychosis in a Predisposed Patient.

Psilocybin is gaining popularity as research shows potential benefits to those with anxiety, depression, and other mental health conditions. Individuals with risk factors for psychosis are typically excluded from such studies, limiting the empiric research of the risks and benefits in vulnerable populations. In the real-world setting, many individuals who seek treatment with psilocybin will have comorbid psychiatric conditions and other factors that predispose them to psychosis. We report a case of a patient with multiple predisposing risk factors, including a history of depression, personality disorder traits, and cannabis use, who experienced a psychotic episode with catatonic features and suicidality after several months of heavy psilocybin use. A review of similar previously published case reports demonstrates a pattern of psilocybin-induced psychosis occurring primarily in individuals with predisposing factors who have consumed either high or repeated doses of the drug. This case report furthers this pattern, which serves as both a warning that psilocybin use is not without risks and reassurance for researchers using much lower doses to treat mental illness.

Psilocybin-assisted neurofeedback for the improvement of executive functions: a randomized semi-naturalistic-lab feasibility study.

Executive function deficits, common in psychiatric disorders, hinder daily activities and may be linked to diminished neural plasticity, affecting treatment and training responsiveness. In this pioneering study, we evaluated the feasibility and preliminary efficacy of psilocybin-assisted frontal-midline theta neurofeedback (NF), a neuromodulation technique leveraging neuroplasticity, to improve executive functions (EFs). Thirty-seven eligible participants were randomized into an experimental group (n = 18) and a passive control group (n = 19). The experimental group underwent three microdose sessions and then three psilocybin-assisted NF sessions, without requiring psychological support, demonstrating the approach's feasibility. NF learning showed a statistical trend for increases in frontal-midline theta from session to session with a large effect size and non-significant but medium effect size dynamical changes within sessions. Placebo effects were consistent across groups, with no tasks-based EF improvements, but significant self-reported gains in daily EFs-working memory, shifting, monitoring and inhibition-showing medium and high effect sizes. The experimental group's significant gains in their key training goals underscored the approach's external relevance. A thorough study with regular sessions and an active control group is crucial to evaluate EFs improvement and their specificity in future. Psilocybin-enhanced NF could offer significant, lasting benefits across diagnoses, improving daily functioning. This article is part of the theme issue 'Neurofeedback: new territories and neurocognitive mechanisms of endogenous neuromodulation'.

Exploring the biocatalysis of psilocybin and other tryptamines: Enzymatic pathways, synthetic strategies, and industrial implications.

Tryptamines play diverse roles as neurotransmitters and psychoactive compounds found in various organisms. Psilocybin, a notable tryptamine, has garnered attention for its therapeutic potential in treating mental health disorders like depression and anxiety. Despite its promising applications, current extraction methods for psilocybin are labor-intensive and economically limiting. We suggest biocatalysis as a sustainable alternative, leveraging enzymes to synthesize psilocybin and other tryptamines efficiently. By elucidating psilocybin biosynthesis pathways, researchers aim to advance synthetic methodologies and industrial applications. This review underscores the transformative potential of biocatalysis in enhancing our understanding of tryptamine biosynthesis and facilitating the production of high-purity psilocybin and other tryptamines for therapeutic and research use.

Moving psychedelic-assisted therapies from promising research into routine clinical practice: Lessons from the field of implementation science.

Psychedelics (e.g., 3,4-Methylenedioxymethamphetamine [MDMA], lysergic acid diethylamide [LSD], psilocybin) are molecules that have the potential to produce rapid therapeutic effects when paired with psychotherapy. Randomized clinical trials of psychedelic-assisted psychotherapy (PAT) have shown promising results for post-traumatic stress disorder (PTSD), depression, and substance use disorders. The U.S. Food and Drug Administration has acknowledged the promise of PAT, signaling potential approval of psilocybin-assisted therapy for depression by 2026. Given this timeline, implementation scientists must engage with PAT researchers, policymakers, and practitioners to think critically about bringing these promising new treatments into routine practice settings while maintaining quality and safety. This commentary aims to initiate a dialogue between implementation scientists and PAT researchers and practitioners on addressing these questions with a lens toward equity. Specifically, we discuss how the field of implementation science can support PAT stakeholders to accelerate the translational process from research into practice, focusing specifically on safety-net settings (i.e., Federally Qualified Health Centers and Veterans Affairs health systems) that serve historically marginalized populations. We use the Reach, Effectiveness, Adoption, Implementation, and Maintenance (RE-AIM) Framework to illustrate five critical areas where implementation science can help move PAT from research into real-world practice. For each RE-AIM dimension, we highlight ways the field of implementation science can contribute tools (e.g., implementation strategies), methodologies (e.g., pragmatic hybrid implementation-effectiveness trials), and approaches (community-based participatory research) for establishing the safety, effectiveness, and accessibility of PAT for historically underserved communities.

Clinical trials of psychedelic-assisted therapy (PAT) have shown promising safety and efficacy results for treating a number of mental health conditions, including post-traumatic stress disorder, depression, anxiety/depression associated with life-threatening illnesses (e.g. cancer), eating disorders, and alcohol and tobacco use disorders. PAT researchers and practitioners must think critically about how to best bring these promising treatments into routine practice settings that are outside the confines of randomized clinical trials. This commentary aims to initiate an ongoing dialogue on how the field of implementation science can contribute tools, methodologies, and approaches to help move PAT from research into practice for historically underserved communities. We use the Reach, Effectiveness, Adoption, Implementation, and Maintenance (RE-AIM) framework to illustrate five critical areas where PAT can help move from randomized control trials into practice. Throughout all the considerations we pose, we highlight how an equity lens is necessary to acknowledge and address harms caused to historically marginalized communities by policies like the "War on Drugs" and advocate that those most in need of PAT receive it first at safety-net health settings like Federally Qualified Health Centers and the Veterans Affairs.

Mechanisms of psilocybin on the treatment of posttraumatic stress disorder.

Posttraumatic stress disorder (PTSD) is a condition that can develop after a traumatic event, causing distressing symptoms, including intrusive re-experiencing symptoms, alterations in mood and cognition, and changes in arousal and reactivity. Few treatment options exist for patients who find conventional psychotherapy and pharmacotherapy to be inaccessible, ineffective, or intolerable. We explore psilocybin as a potential treatment option for PTSD by examining the neurobiology of PTSD as well as psilocybin's mechanism of action. Based on both pharmacodynamic and psychoanalytic principles, psilocybin may be an underexplored treatment option for patients with PTSD, though further research is required.

Comparing psilocybin to metformin as neuroprotective agents against Parkinson's dementia: A systematic review of evidence and efficacy.

BACKGROUND & AIM: Treatment of Parkinson's disease (PD) has remained largely unchanged and focuses primarily on symptomatic relief through activation of dopaminergic pathways. Currently, there are no proven prophylactic approaches to the prevention of PD. This systematic review seeks to compare two separate compounds, metformin (MTF) and psilocybin, as potential prophylactic therapeutics against the development of PD. METHODS: The authors conducted a systematic review focusing on primary studies that test these compounds on cell and animal models to determine if they might have any neuroprotective or neuroplastic effects. RESULTS: The results of this review found that MTF may halt the progression of diseases such as PD through multiple mechanisms including reduced oxidative stress at the level of the mitochondria, thereby reducing α-synuclein related damage. Psilocybin, on the other hand, may increase repair of damaged neurons through psychoplastogenic activation of serotonergic pathways, particularly 5-HT2A receptor activation, ultimately increasing the release of brain derived neurotropic factor (BDNF) and the reduction of α-synuclein accumulation. CONCLUSION: Implications of this study include a need for further research in off-label use of MTF as well as further research into serotonergic compounds such as psilocybin for the treatment and prevention of neurodegenerative diseases.

Probing the functional magnetic resonance imaging response to psilocybin in functional neurological disorder (PsiFUND): study protocol.

Background: Functional neurological disorder (FND) is a common cause of neurological symptoms including paralysis, seizures, and movement disorders. It is often debilitating, is associated with high health and social care costs, and can have a poor prognosis. Functional magnetic resonance imaging (fMRI) has suggested FND is a multi-network disorder; the default mode network (DMN) may be specifically implicated. Converging evidence suggests that other variable mechanisms including dissociation, interoception, and motor agency may be differentially abnormal in people with FND. Psychedelics are currently under investigation for numerous neuropsychiatric disorders and have been shown to disrupt functional networks such as the DMN. Administering psychedelics to people with FND will help us to probe mechanistic theories of the disorder. Protocol: In this open-label neuroimaging study, we will administer 25mg oral psilocybin with psychological support to people with chronic FND (target n = 24). Participants will undergo resting-state and task-based (Libet's clock, a measure of motor agency) fMRI sequences which will be compared in a pre-post manner. Additional mechanistic outcomes including measures of interoception (heartbeat tracking task), somatisation, illness perceptions, imaginative suggestibility, and dissociation will be collected. Data on expectancy, preparedness, and subjective experience of the psychedelic experience will also be gathered. Participants will be followed up for three months following psilocybin administration. fMRI changes in networks such as the DMN will be analysed using seed-based approaches, and additional exploratory analysis of resting-state imaging will take place. Discussion: The study will help us to probe the mechanisms thought to potentially underpin FND. As the first modern study of psychedelics in FND, it will also help us to understand whether psychedelic administration alongside psychological support might be safe and feasible in this patient population.

Neuroprotective effects of psilocybin in a rat model of stroke.

BACKGROUND: Psilocybin is a psychedelic 5HT2A receptor agonist found in "magic mushrooms". Recent studies have indicated that 5HT2A agonists, such as dimethyltryptamine, given before middle cerebral artery occlusion (MCAo), improve staircase behavior, increased BDNF expression, and reduce brain infarction in stroke rats. The objective of this study is to determine the protective effect of psilocybin in cellular and animal models of stroke. METHODS: Adult male and timed-pregnant Sprague-Dawley rats were used for this study. The neural protective effects of psilocybin were determined in primary rat cortical neurons and adult rats. Rats were subjected to a 60-min middle cerebral artery occlusion. Brain tissues were collected for histological and qRTPCR analysis. RESULTS: Psilocybin reduced glutamate-mediated neuronal loss in rat primary cortical neuronal cultures. Psilocybin-mediated protection in culture was antagonized by the BDNF inhibitor ANA12. Pretreatment with psilocybin reduced brain infarction and neurological deficits in stroke rats. Early post-treatment with psilocybin improved locomotor behavior, upregulated the expression of MAP2 and synaptophysin, and down-regulated the expression of IBA1 in the stroke brain. ANA12 significantly attenuated psilocybin-mediated reduction in brain infarction and improvements in locomotor behavior. CONCLUSIONS: Psilocybin reduced brain infarction and improved locomotor behavior in stroke rats; the protective mechanisms involve regulating BDNF expression. Our data support a novel therapeutic approach of psilocybin in stroke.

Latin American adults who regularly use macrodoses of psychedelics: a cross-sectional study.

Psychedelics have a complex history marked by traditional use among indigenous cultures, early scientific interest, and subsequent prohibition. Despite their classification as controlled substances, recent decades have witnessed a resurgence of research into their therapeutic potential for various mental health conditions. However, most studies have focused on controlled clinical settings, leaving a significant gap in understanding how these substances are used in naturalistic contexts, particularly in Latin America. This study investigates the regular use of macrodoses of psychedelics among Latin American adults. We aimed to characterize the sociodemographic profiles, consumption practices, and subjective effects experienced by individuals who use psychedelics regularly. Data were collected via an online survey from 4,270 participants across several Latin American countries. Results indicated a diverse user base with varied motivations, predominantly psychological and spiritual well-being. The most frequently used substance was psilocybin mushrooms, with significant associations found between demographic variables and specific psychedelics used. The study provides new insights into the naturalistic use of psychedelics in Latin America, highlighting the need for informed, safe, and legal use frameworks.

The Second Methylation in Psilocybin Biosynthesis Is Enabled by a Hydrogen Bonding Network Extending into the Secondary Sphere Surrounding the Methyltransferase Active Site.

The Psilocybe cubensis SAM-dependent methyltransferase, PsiM, catalyzes the last step in the biosynthesis of psilocybin. Likely evolved from monomethylating RNA methyltransferases, PsiM acquired a key amino acid exchange in the secondary sphere of the active site, M247N, which is responsible for its capacity to dimethylate. Two variants, PsiMN247M and PsiMN247A, were generated to further examine the role of Asn247 for mono- and dimethylation in PsiM. Herein, we present the kinetic profiles of both variants and crystal structures at resolutions between 0.9 and 1.0 Å. Each variant was crystallized as a ternary complex with the non-methylated acceptor substrate, norbaeocystin and S-adenosyl-l-homocysteine, and in a second complex with the cofactor analog, sinefungin, and the monomethylated substrate, baeocystin. Consistent with the inability of the variants to catalyze a second methyl transfer, these structures reveal catalytically non-productive conformations and a high level of disorder of the methylamine group of baeocystin. Additionally, both variants exhibit destabilization in the ß5-ß7 sheets and a conserved ß-turn of the core Rossmann fold, causing 20-fold reduced substrate binding and 2-fold lower catalytic efficiency even with norbaeocystin.

The association between study design and antidepressant effects in psychedelic-assisted therapy: A meta-analysis.

Different study designs of psychedelic trials may impact the blinding and expectance, leading to biased treatment effects. This study aimed to examine the association between antidepressant efficacy and study designs in psychedelic trials. Six databases were systematically searched. Eligible trials were required to investigate the efficacy of psychedelics (psilocybin, lysergic acid diethylamide [LSD], 3,4-Methylenedioxymethamphetamine [MDMA], and ayahuasca) in adult patients with depressive symptoms. We only considered oral psychedelic-assisted therapy without concomitant use of antidepressants. The primary outcome was the change in depressive symptoms. There were five study designs of psychedelic trials, including non-active-drug-as-placebo, active-drug-as-placebo, waitlist-as-control, fixed-order, and pre-post designs. In non-active-drug -as-placebo design, psilocybin (k = 4, Hedges' g [g] = 0.87, 95 % confidence intervals[CIs] = 0.58 to 1.16) and MDMA (k = 2, g = 0.65, 95%CIs = 0.26 to 1.05) were associated with large and medium effect sizes, respectively. In active-drug-as-placebo design, both psilocybin (k = 2, g = 0.71, 95%CIs = -0.01 to 1.43) and MDMA (k = 3, g = 0.53, 95%CIs = -0.23 to 1.28) were not statistically significant. In pre-post single-arm (k = 3, g = 2.51, 95%CIs = 1.00 to 4.02) and waitlist-as-control (k = 1, g = 2.88, 95%CIs = 1.75 to 4.00) designs, psilocybin showed a large effect size of antidepressant effect. Ayahuasca also showed a large effect size in both pre-post (k = 2, g = 1.88, 95%CIs = 1.18 to 2.57) and non-active-drug-as-placebo (k = 1, g = 1.60, 95%CIs = 0.84 to 2.36) designs. LSD was associated with a significant antidepressant effect only in non-active-drug-as-placebo design (k = 1, g = 1.49, 95%CIs = 0.80 to 2.17) but not in active-drug-as-placebo design (k = 1, g = 0.44, 95%CIs = -0.90 to 1.78). The antidepressant effects of psychedelics may be overestimated in studies with pre-post single-arm, non-active-drugs-as placebo, and waitlist-control designs. Restricted sample size, difficulty with establishing blinding for participants, and over expectancy limit the estimation of the antidepressant effect of psychedelic-assisted therapy.