Optimal dosing for psilocybin pharmacotherapy: Considering weight-adjusted and fixed dosing approaches

Interest in classic psychedelics has increased because these compounds, which act primarily as serotonin 2A receptor (5-HT2A R) agonists, show therapeutic promise for mood and substance use disorders. Psilocybin, a prodrug converted to psilocin, has produced moderate-to-large effect sizes in trials combined with supportive counselling and has yielded persistent benefits after one to a few administrations. Most clinical studies to date have used body weight-adjusted dosing (for example 30 mg/70 kg), but a fixed-dose approach would be operationally simpler and less costly; it remains uncertain whether weight-adjustment is necessary to achieve comparable subjective or therapeutic effects across people of differing body mass. Garcia-Romeu and colleagues set out to examine whether acute subjective effects of psilocybin vary as a function of body weight under both weight-adjusted and fixed-dose scenarios. Using pooled post hoc data from ten prior studies, the investigators focused on three specific questions: whether subjective effects vary with body weight at 20 mg/70 kg, at 30 mg/70 kg, and when participants received an absolute dose of about 25 mg (to approximate a fixed dosing regimen being used in registration trials). The analysis concentrates on acute subjective measures (mystical-type experiences, challenging experiences, and peak intensity) because these effects have been linked in prior work to subsequent therapeutic outcomes.

This report is a secondary, post hoc analysis of data pooled from ten studies conducted at Johns Hopkins between 2001 and 2018. The pooled sample comprised adults (total N = 288) who had received body weight-adjusted psilocybin doses within the 20 to 30 mg/70 kg range; institutional review board approval and informed consent were recorded in the original trials. Participants came from diverse study populations including healthy volunteers (with and without prior hallucinogen use), cigarette smokers seeking cessation, patients with life-threatening cancer-related distress, meditators, religious professionals, and people with major depressive disorder. Acute subjective outcome measures were the 30-item Mystical Experience Questionnaire (MEQ30), the 26-item Challenging Experience Questionnaire (CEQ), and a single-item peak intensity rating (0–4). These instruments were administered at the end of the psilocybin session day, approximately 7 hours after capsule ingestion. A “complete mystical experience” on the MEQ30 was defined as scoring at least 60% of the maximum on each of the four subscales. Analyses used multivariate regression models in which the endogenous variables were MEQ30 total, CEQ total, and intensity (each standardised to unit variance), and exogenous predictors included age, race (coded white vs not white), sex, and body weight. Models were fit separately for three pooled subsets: sessions at 20 mg/70 kg (n = 120), sessions at 30 mg/70 kg (n = 182), and sessions with an absolute dose between 23 and 27 mg (inclusive; n = 103) to approximate a 25 mg fixed dose. Because absolute dose and weight are perfectly collinear in the weight-adjusted subsets, absolute dose was not included as an exogenous predictor in those models. Additional sensitivity models replaced weight with body mass index (BMI) for the 23–27 mg subset and included dummy codes for study membership; results of those supplemental models are reported as not substantively different. A Bonferroni-corrected alpha of p < 0.004 (0.05/12 comparisons) was used for statistical significance. Analyses were performed using the lavaan package in R and plotted with GraphPad Prism.

Across the pooled datasets, no statistically significant associations emerged between participant demographics (age, race, sex, or body weight) and the acute subjective measures of psilocybin effects after correction for multiple comparisons. In the 20 mg/70 kg subset (n = 120), participants had a mean (SD) age of 42.2 (12.6) years and 53% were female. Body weight ranged from 49 to 111 kg, producing absolute doses from 14.0 to 31.6 mg. On average MEQ30 scores and intensity ratings were moderately high while CEQ scores were low to moderate. Multivariate regression models did not show significant relationships between MEQ30 total, CEQ total, or intensity and any demographic predictor, including weight. Models that included dummy variables for study membership produced substantively similar results, with one trial contributing to somewhat attenuated challenging-effect estimates (detailed in supplemental materials). In the 30 mg/70 kg subset (n = 182), mean (SD) age was 42.7 (12.2) years and 56% were female. Body weights spanned 49 to 113 kg, with absolute doses from 21.0 to 48.6 mg. Average MEQ30 scores and intensity ratings were high and CEQ scores remained low-to-moderate. Again, no significant associations were found between subjective measures and demographic variables, including weight. Inclusion of study dummy codes yielded similar outcomes, with one study showing attenuation of intensity ratings relative to others (see supplemental materials). For the subset receiving absolute doses between 23 and 27 mg (n = 103), body weights ranged from 54 to 94 kg. No significant associations were observed between MEQ30 total, CEQ total, or intensity and age, race, sex, weight, or BMI. Sensitivity models using BMI or study membership did not alter these conclusions. Across the full pooled range of body weights (49 to 113 kg), the analyses provided no evidence that body weight significantly affected the measured acute subjective effects of psilocybin. The extracted text does not report exact regression coefficients, confidence intervals, or p-values for individual predictors in the main text; the authors note no associations reached the corrected significance threshold.

Garcia-Romeu and colleagues interpret these pooled post hoc findings as showing no meaningful relationship between participant body weight and acute subjective effects of psilocybin across weight-adjusted dosing at 20 mg/70 kg and 30 mg/70 kg and, in a smaller subsample, for absolute doses around 25 mg. Because acute mystical-type, intensity, and challenging experiences have been linked in earlier studies to therapeutic outcomes, the lack of weight-related differences suggests that fixed dosing may provide comparable subjective effects and, by implication, comparable therapeutic potential to weight-adjusted dosing. The investigators caution, however, that the present analyses are not definitive evidence favouring one dosing strategy. Analyses were post hoc and used data from studies that originally administered weight-adjusted doses, so direct prospective comparison of fixed versus weight-adjusted regimens was not performed. Most participants were healthy volunteers and the subsample approximating a 25 mg fixed dose was smaller, which the authors acknowledge may have limited statistical power to detect modest effects. Racial homogeneity (predominantly white participants) and other sampling constraints limit generalisability. The findings are presented as broadly consistent with a recent pharmacokinetic simulation suggesting a 25 mg fixed oral dose may yield psilocin exposures similar to a 0.3 mg/kg weight-adjusted dose, and with open-label reports of clinical improvement following fixed 25 mg dosing. The authors note trend-level indications—such as marginally higher self-reported challenging effects in women and a dose-related trend across 20 to 30 mg—that merit further investigation. They also emphasise substantial inter-individual variability in response that is independent of body weight, pointing to other influences (for example personality, acute mental state, environment or “set and setting”, genetic factors) that were not examined here but could affect safety and efficacy. Finally, the authors recommend prospective randomised studies comparing fixed and weight-adjusted dosing in clinically defined, more diverse populations and suggest incorporating pharmacogenetic and neurobiological assessments, as well as careful study of set and setting, to establish optimal dosing practices for therapeutic psilocybin administration.