Single-Dose Psilocybin Treatment for Major Depressive Disorder: A Randomized Clinical Trial

Interest in the therapeutic potential of psilocybin for major depressive disorder (MDD) has grown rapidly, driven by both limitations of existing antidepressant treatments and early studies suggesting rapid, durable antidepressant effects after single doses. Raison and colleagues note, however, that many earlier trials suffered from limitations including small samples, open-label or waitlist comparators, likely functional unblinding of outcome raters, and incomplete reporting of adverse events. Short primary end points in several recent trials also left unanswered questions about the durability of benefit for an often chronic condition such as MDD. To address these gaps, the investigators ran a multicentre, randomized, double-blind Phase II trial comparing a single 25-mg oral dose of synthetic psilocybin with an active placebo (100 mg niacin), each delivered within a standardised psychological support programme. The study sought to characterise the magnitude, timing and 6-week durability of antidepressant effects, and to evaluate safety using centralised blinded raters and a prespecified assessment schedule up to day 43 after dosing.

This was a randomised, two-arm Phase II clinical trial conducted at 11 US sites between December 2019 and June 2022. Adults aged 21–65 meeting DSM-5 criteria for MDD with a current episode of at least 60 days were eligible if they had a central rater MADRS score ≥28 at screening and showed ≤30% improvement during a 7–35 day period that allowed psychotropic medication tapering. Key exclusions included personal or first‑degree family history of psychosis or mania, moderate/severe alcohol or drug use disorder, active suicidal intent or plan, recent or heavy prior psychedelic use (use in past 5 years or >10 lifetime uses), and inability to discontinue prohibited psychotropic medications. There were no exclusions based on number of prior depressive episodes or maximum symptom severity, aside from prior invasive neuromodulation procedures. After screening and preparatory sessions, participants were randomised 1:1 using permuted blocks stratified by site to receive a single oral dose of either 25 mg psilocybin or 100 mg niacin; randomisation and bottle assignment were managed centrally to preserve blinding. Both treatments were administered within an identical, protocolised "set and setting" programme that included 6–8 hours of preparatory sessions with two facilitators, a 7–10 hour supervised dosing session (participants encouraged to wear eyeshades and listen to a curated playlist), and about 4 hours of postdose integration. Lead facilitators were doctoral-level psychologists or physicians; co‑facilitators had at least a bachelor's degree in a mental‑health related field. Facilitators completed protocol-specific training, although fidelity monitoring was not reported. The primary outcome was change in central rater‑assessed Montgomery–Åsberg Depression Rating Scale (MADRS; 0–60) from baseline to day 43. The key secondary outcome was change in MADRS from baseline to day 8. Additional secondary outcomes included change in Sheehan Disability Scale (SDS) from baseline to day 43 and proportions meeting prespecified definitions of sustained response (≥50% MADRS reduction at days 8, 15, 29 and 43) and sustained remission (MADRS ≤10 at those same visits). Centralised telephone raters, blinded to treatment and visit, conducted MADRS assessments to reduce functional unblinding. Safety monitoring collected adverse events (AEs) from enrolment through day 43, with solicited items including suicidal ideation (C‑SSRS), elevated blood pressure/heart rate requiring treatment, headache, nausea, and visual perceptual effects. The trial was powered for a day 43 primary end point after an FDA recommendation change; a sample size of 100 was chosen to provide high power for the day 43 and day 8 comparisons under assumed effect sizes. The primary analysis used an intent‑to‑treat (ITT) de facto estimand including all randomised participants analysed by assigned group. Continuous outcomes were analysed with mixed‑effects models for repeated measures adjusted for baseline score, site, sex and treatment‑resistant depression (TRD) status, with time treated categorically and a treatment×time interaction. Sensitivity analyses used multiple imputation. Sustained response/remission were analysed with logistic regression adjusted for sex and TRD. A sequential hypothesis testing procedure controlled the familywise alpha for primary and specified secondary endpoints.

Of 1529 prescreened individuals, 347 consented, and 104 were randomised and received study drug (51 assigned to psilocybin, 53 to niacin). Due to a dispensing error one participant assigned to psilocybin received the comparator, yielding 50 participants who actually received psilocybin and 54 who received niacin in the safety population. Baseline characteristics were broadly balanced: mean age ~41 years, 50% women, 89% White, prior lifetime psychedelic use reported by about 20% of each group, median duration of current episode measured in many months (median 53 weeks for psilocybin, 81 weeks for niacin), and a low prevalence of TRD (approximately 12.5% of the ITT sample). By day 43, 1 participant in the psilocybin group and 9 in the niacin group had withdrawn or been lost to follow‑up; no withdrawals were attributed to AEs. On the primary outcome, psilocybin produced greater reduction in depressive symptoms than niacin from baseline to day 43: mean difference in MADRS change −12.3 (95% CI −17.5 to −7.2), P < .001. The key secondary end point at day 8 also favoured psilocybin: mean difference −12.0 (95% CI −16.6 to −7.4), P < .001. Similar between‑group differences were observed at days 15 and 29 and in multiple imputation sensitivity analyses. More participants randomised to psilocybin achieved sustained depressive symptom response across the postdose visits, although the extracted text does not provide the complete ITT counts for sustained response. In the per‑protocol population (n = 81), results were similar; here the difference in sustained remission reached statistical significance (12/44 [27%] psilocybin vs 3/36 [8%] niacin; adjusted absolute difference 18.9% [95% CI 3.0%–34.9%]; OR 4.0 [95% CI 1.0–15.7]; P = .04). Exploratory outcomes showed improvements with psilocybin in clinician global severity, self‑reported depressive and anxiety symptoms, and quality of life, with no evidence of increased emotional blunting. Safety analyses showed higher rates of adverse events in the psilocybin group. Across the study through day 43, 44/50 (88%) participants receiving psilocybin and 33/54 (61%) receiving niacin reported at least one AE. From dosing day through day 9, 41/50 (82%) psilocybin participants experienced at least one drug‑related treatment‑emergent AE (TEAE) versus 24/54 (44%) in the niacin arm (absolute difference 38% [95% CI 20.6%–41.3%]; relative incidence 1.8). Severe drug‑related AEs through day 9 occurred in 4/50 (8%) psilocybin recipients (examples included migraine, headache, illusion, panic attack with paranoia) and in none of the niacin recipients. The most common solicited events were headache (33/50 [66%] vs 13/54 [24%]; difference 42% [95% CI 27.3%–57.6%]; RI 2.7) and nausea (24/50 [48%] vs 3/54 [6%]; difference 42% [95% CI 24.5%–59.3%]; RI 8.6). Visual perceptual effects were reported on the dosing day by 22/50 (44%) psilocybin participants and after the dosing day by 3/50 (6%); all resolved by study end. No serious treatment‑emergent adverse events were reported, and no instances of clinically confirmed active suicidal behaviour occurred during the trial; suicidal ideation increases were reported in one psilocybin and five niacin participants between baseline and end of trial. No clinically significant changes in vital signs or laboratory tests were observed.

Raison and colleagues interpret these results as evidence that a single 25‑mg dose of psilocybin administered with standardised psychological support produced statistically and clinically meaningful reductions in depressive symptoms compared with an active niacin placebo, with improvements apparent by day 8 and maintained through the 6‑week primary end point. The reported between‑group difference of 12.3 MADRS points at day 43 is larger than typical active‑placebo differences cited in the depression literature and the absolute MADRS reduction in the psilocybin group was described by the authors as exceeding thresholds commonly used to indicate substantial clinical improvement. Psilocybin also improved psychosocial functioning (SDS) and several exploratory measures including anxiety and quality of life, and did not increase emotional blunting. The investigators note several caveats. First, they acknowledge the likelihood of some degree of functional unblinding from psilocybin's acute psychoactive effects, and that the study did not formally assess blinding success; centralised remote raters were used to mitigate this risk but may not fully remove it. Second, the choice of niacin as an active placebo could have affected placebo responses; the authors observe that placebo response in this trial was larger than with an inert placebo in other studies but smaller than typical daily‑pill placebo rates. Third, the 6‑week follow‑up provides longer observation than some recent trials but still leaves open questions about longer‑term durability of effect, especially given the low proportion of participants with treatment‑resistant depression in this sample. Fourth, fidelity to the psychosocial support protocol was not assessed, so between‑participant variability in therapeutic support cannot be excluded as a contributor to outcomes. Finally, the sample lacked racial and ethnic diversity and was generally of higher socioeconomic status, limiting generalisability; the authors emphasise the need for future trials that actively recruit underrepresented groups. In terms of safety, psilocybin was generally well tolerated with most AEs mild or moderate and concentrated around the dosing period, but it was associated with higher overall and severe AE rates compared with niacin. The authors caution that psychedelics may produce adverse effects not fully captured by standard scales and stress the importance of continued careful safety monitoring in future studies. They recommend further large‑scale, longer‑term, and more diverse trials, consideration of alternative comparators that better preserve blinding (for example ketamine), and evaluation of the contribution of psychological support to clinical outcomes.

In this multicentre, randomised Phase II trial, a single 25‑mg dose of psilocybin given with protocolised psychological support produced clinically and statistically significant reductions in depressive symptoms and improvements in functional disability compared with a 100‑mg niacin active placebo at 6 weeks, without any serious treatment‑emergent adverse events. Psilocybin was associated with higher rates of overall, solicited and severe AEs, most occurring around the dosing session and resolving by study end. The authors conclude these results add to evidence that psilocybin, when administered with psychological support, may hold promise as a novel intervention for MDD, while underscoring the need for longer‑term, larger, and more diverse studies and for improved assessment of blinding and psychological‑support fidelity.