This was a longitudinal, mixed-methods, open-label study examining the acute and sustained effects of a structured psilocybin-assisted therapeutic workshop (Osman et al., 2025). Data collection was embedded within a 7-day retreat-based model, conducted at a private therapeutic retreat center in Negril Jamaica, where psilocybin and psilocin are not scheduled. The study received ethical approval from the University of Ottawa Research Ethics Board (REB approval #H-09-22-8424). Researchers obtained written informed consent from all participants after explaining the study's risks, benefits, and procedures. The study was conducted in strict accordance with APA and Declaration of Helsinki ethical guidelines.

Target psilocybin doses were determined based on the quantified concentration of psilocybin (C_ps​ in mg/g) in each homogenized mushroom batch. Following homogenization, a representative aliquot with the extract of the powdered Psilocybe cubensis material was analyzed photometrically to yield the psilocybin concentration C_ps (mg of psilocybin per gram of dried material). The specific dry weight of mushroom material (m material) needed to achieve the total target psilocybin dose (D_ps) specific dry weight of mushroom material (m material) needed to achieve the total target psilocybin dose (D_ps) for each participant was then calculated according to the formula:

The workshop was staffed 24/7 by a clinical team to ensure participant safety, maintaining a ratio of 6 support staff/clinicians per session. The team included a Medical Doctor (Anesthesiologist), two clinical Psychologists, a Nurse Practitioner, and three experienced Assistants/Facilitators.

Quantitative data were collected at three time points: Baseline (Pre-Intervention), Post-Intervention (Acute/Post-Workshop), and Follow-up (approximately 6 weeks post-session). All quantitative measures were validated self-report instruments, including those specific to racial trauma and general well-being, and are detailed in a companion methodology paper (blinded for review). In addition, an optional component included post-session and follow-up semi-structured interviews to gather rich, qualitative phenomenological data on the psilocybin experience and its long-term impact.

Psilocybin is unique in being regarded as having one of the lowest risks of acute toxicity from overdose, which is reflected in its high therapeutic index (Dodd et al., 2023). Preclinical work determined the LD50 for intravenous psilocybin above 250mg/kg (Cerletti 1958). Based on this, the theoretical lethal dose has been estimated to be roughly 1000 times an effective dose, a level considered physically impossible to ingest via psilocybin-containing mushrooms (Gable 2004). Consistent with this, only one fatal overdose has ever been documented, a medically compromised heart-transplant patient with pre-existing cardiac disease (Lim et al., 2012), indicating that lethal toxicity is extraordinarily rare. Experimental studies in animals showed no chromosomal damage, teratogenicity, or significant organ toxicity even at high doses (Cerletti 1958, Van Went 1978). In humans, psilocybin has not been associated with physiological toxicity, respiratory depression, or cardiovascular mortality. Adverse effects are typically limited to transient nausea, anxiety, or mild increases in blood pressure and heart rate (Ross et al., 2016, Griffiths et al., 2016). The most common side effect in controlled trials is transient, delayed-onset headache, which resolves within 24 hours (Johnson et al., 2012). Larger population analyses and regulatory evaluations concur that psilocybin’s acute and chronic toxicity are low, with negligible dependence potential and minimal public-health impact (van Amsterdam et al., 2011, Nutt et al, 2010). While therapeutic doses typically range from 15 to 30 mg, the estimated lethal dose is reported to be 500 times higher, at 6000 mg (Dodd et al., 2023). Any fatality is more likely associated with accidental mushroom misidentification or dangerous behavior resulting from severely disrupted judgment (Dodd et al., 2023). Moreover, a comprehensive review of abuse liability concluded that psilocybin produces no physical dependance, minimal reinforcement, and very limited self-administration behavior, supporting its classification as a substance with exceptionally low abuse potential. (Johnson et al., 2018)

Safety Profile of Psilocybe cubensis

From both ethnomycological and modern clinical perspectives, Psilocybe cubensis is considered among the safest psychoactive natural products known. Across several decades of human use, ranging from ceremonial to research and retreat settings, there have been no verified cases of fatal toxicity, organ damage, or physiological dependence attributable to P. cubensis itself (Jo et al., 2014; Woo-Sik et al., 2014). Its principal psychoactive constituents, psilocybin and psilocin, exhibit a wide safety margin and minimal systemic toxicity, but the mushroom matrix as a whole is also physiologically benign: it lacks hepatotoxic compounds, does not produce significant autonomic instability, and shows no evidence of reinforcing or withdrawal phenomena (Cohen et al., 2025).

This study's reliance on chemical quantification, rather than mass, is supported by clinical consensus which emphasizes the inherent variability of psilocybin content in natural fungal products (MacCallum et al., 2022). Although the fixed 25 mg dose has become the industry benchmark and relies on standardized potency and reproducibility when synthetic psilocybin is used, our findings demonstrate that high-precision dosing can be achieved even with dried mushroom sources (Goodwin et al., 2025)

Specifically, chemical quantification revealed substantial variation in psilocybin concentration across the analyzed Psilocybe cubensis strains. The first batch, which was a 1:1 homogenized mixture of the Fat Mac and Cambodian strains, tested at 6.07 mg/g, whereas the single-strain Monster Mac batch used in the second session contained 15.7 mg/g of psilocybin. This represents a 2.6-fold difference in potency between batches derived from the same cultivator under comparable environmental conditions.

Such variability underscores the inadequacy of weight-based dosing for natural products and emphasizes the necessity of chemical standardization before administration. Without quantification, participants could easily have received doses differing by more than double in active compound, leading to unpredictable pharmacological and experiential outcomes. These findings align with previous analytical reports demonstrating both inter- and intra-strain variability in P. cubensis potency and emphasize the importance of batch-specific measurement for reproducible and safe psilocybin-assisted interventions. By achieving high-precision dosing, we ensured participants received the targeted therapeutic exposure. Future considerations for patient safety may also need to account for individual variability in sensitivity, which has been linked to factors such as 5HT2A​ receptor polymorphisms and liver/gastric function (MacCallum et al., 2022).

The rationale for administering two distinct doses (Session 1: ∼20 mg; Session 2: ∼56 mg) was based on well-established pharmacological evidence of rapid tolerance, or tachyphylaxis, following serotonergic psychedelic exposure. Psilocybin is rapidly metabolized to psilocin, its active form, which acts primarily as a partial agonist at cortical and subcortical 5-HT2A receptors, key mediators of sensory integration, cognition, and self-referential processing (Dodd et al., 2023). After acute activation by psilocin, these receptors undergo desensitization and internalization, a β-arrestin–mediated process typical of G-protein-coupled receptors following overstimulation (Wallach et al. 2023). However, preclinical evidence indicates that psychedelic tolerance is not exclusively ß-arrestin2-dependant: repeated or even single exposures to 5-HT2A agonists such as DOI or LSD induce marked receptor down-regulation in the frontal cortex and diminished behavioral responsiveness (head-twitch response, HTR) persisting for more than 24h, even in ß-arrestin-2 knockout mice (de la Fuente Revenga et al., 2022, Smith et al., 2014) This transient reduction in functional 5-HT2A receptor availability leads to blunted subjective and neurophysiological responses to subsequent doses administered within a short interval (typically 1–7 days). Classic human experiments demonstrated cross-tolerance between psilocybin, LSD, and mescaline, confirming a shared receptor-level mechanism (Isbell et al., 1961, Wolbach et al., 1962, Buchborn et al., 2016). Interestingly, tolerance to subjective effects often develops faster than tolerance to physiological markers such as blood pressure or cortisol changes, suggesting selective desensitization within higher-order cortical circuits (Vollenweider & Kometer 2010, Buchborn et al., 2016).

To achieve a subjectively and pharmacologically comparable intensity during the second session, the dose was increased by a factor of approximately ×2.8 (from ~ 20 mg to ~ 56 mg). This strategy is consistent with both preclinical and clinical observations of short-term receptor downregulation after psilocybin exposure and was supported by participants’ phenomenological reports indicating similar experiential intensity across sessions.

Following the first session, we conducted a structured case review integrating behavioral observation, thought-content patterns, markers of cognitive/affective rigidity (e.g., perseverative, highly rule-bound or over-controlled styles), and participants’ qualitative phenomenology. In three individuals this synthesis suggested a comparatively blunted or overly “controlled” psychedelic response despite adequate set/setting and the absence of adverse effects. Given (i) short-interval tachyphylaxis expected after serotonergic psychedelic exposure, (ii) known inter-individual variability in 5-HT2A-mediated sensitivity, and (iii) the therapeutic aim of transiently relaxing over-constrained high-level priors to enable psychological flexibility and emotional processing, we pre-planned a cautious, clinical within-protocol escalation for the second session in these three participants. This individualized titration is consistent with a precision-dosing framework: the goal is not to maximize intensity per se, but to achieve comparable pharmacological engagement and therapeutic window across heterogeneous participants, especially those whose rigid cognitive style may require slightly higher receptor engagement to reach a similar depth of experience while maintaining safety.

Beyond psilocybin and psilocin, Psilocybe species biosynthesize β-carboline monoamine oxidase inhibitors (e.g., harmane, harmine) and a suite of other indole alkaloids (Blei et al., 2020). Recent genomic and metabolomic analyses have also identified active terpene synthases in P. cubensis, suggesting additional terpenoid constituents in the native matrix (Schäfer et al., 2023). These compounds could theoretically modulate psilocybin’s pharmacokinetics or cortical signaling via “entourage” effects (Russo, 2011) and may help explain consistent anecdotal reports of strain-dependent phenomenology in P. cubensis.

However, rigorous human data are scarce, historically, even the psilocybin content of natural preparations was rarely standardized, so most claims remain speculative.

Notably, preclinical work in rodents has begun to delineate the pharmacodynamic differences between isolated psilocybin and full-spectrum mushroom preparations. In the SAPAP3 rodent model of compulsive behavior, Brownstien et al. (2024) demonstrated that a Psilocybe cubensis extract significantly reduced self-grooming at one fifth the psilocybin equivalent dose required for synthetic psilocybin, while producing longer-lasting behavioral remission and broader up-regulation of synaptic plasticity markers in the orbitofrontal cortex and striatum. Moreover, metabolomic profiling revealed distinct biochemical signatures between synthetic psilocybin and the full-mushroom extract treated animals, supporting a pharmacologically unique effect of the native fungal matrix beyond psilocybin alone. These findings suggest that full-spectrum preparations may exert enhanced or prolonged neuroplastic and behavioral effects, warranting further translational investigation into their mechanistic underpinnings and therapeutic implications (Brownstien et al., 2024).

This pragmatic model, combining chemical quantification, dose normalization, and thoughtful retreat design, offers a potential bridge between clinical precision and real-world practice. It may also improve participant safety and data reliability in naturalistic psilocybin studies.

Future research efforts are addressing the issue of tachyphylaxis through the development of next-generation psychedelic-like molecules. For instance, novel compounds such as 2-Br-LSD have been shown in rodent models to prevent the development of tachyphylaxis but also lack the hallmark hallucinogenic effects seen with psilocybin and LSD (Lewis et al., 2023). This absence of tolerance and reduced psychoactivity is hypothesized to stem from the drug's weak recruitment of the 5HT2A​ receptor's β-arrestin2 pathway, (Lewis et al., 2023). This mechanism may provide an alternative therapeutic strategy: it could enable more frequent dosing and significantly reduce the need for the intensive clinical supervision required by a non-ordinary state of consciousness, while still retaining the neuroplastic and antidepressant benefits observed in preclinical models.

To quantitatively assess the subjective effects of psilocybin, researchers can employ the Mystical Experience Questionnaire (MEQ30) (Barrett et al., 2015). The MEQ30 is a validated psychometric instrument designed to measure the depth and quality of the psychedelic experience across four core dimensions: positive mood, transcendence of time/space, ineffability, and mystical quality (Strickland et al., 2024). Scores on the MEQ30 have been repeatedly correlated with long-term therapeutic outcomes across various studies, making it a tool which can link pharmacological dose to clinical efficacy. In this first cohort we were unable to employ this scale however we have plans to do so in future studies. Our primary focus here was on establishing methodological and contextual safety. Future research efforts are required to establish a precise dose-response curve for this specific population and setting, which will necessitate the inclusion of validated psychometric tools such as the MEQ30 to measure experiential outcomes and fully map the pharmacological profile of the intervention.