Written by Austin Mao · Last updated: October 2026 · 8 cited studies.
Background: the depression treatment ceiling
About 21 million U.S. adults experience a major depressive episode in any given year, according to the National Institute of Mental Health. The first-line treatments, selective serotonin reuptake inhibitors and structured psychotherapy, work for many people, but not for all.
The STAR*D study, the largest naturalistic trial of antidepressant sequencing, originally reported a cumulative remission rate of about 67% after up to four sequential trials. That figure is contested: a 2023 patient-level reanalysis reported 35% (Pigott et al., BMJ Open 2023), which the STAR*D investigators dispute. Whichever figure is used, the headline number obscures the lived reality: high dropout, side-effect burden, and weeks-to-months between trials. Treatment-resistant depression is most often defined as failure of at least two adequate antidepressant trials at therapeutic dose and duration.
That gap, between current first-line care and durable remission for people who do not respond, is the clinical question psilocybin trials have been designed to answer.
Mechanism: rapid plasticity, not slow monoamine adjustment
SSRIs adjust serotonin reuptake gradually; clinical effect typically emerges over four to six weeks. Psilocybin acts at a different timescale and through a different downstream pathway. After conversion to psilocin, it is a partial agonist at serotonin 2A receptors (5-HT2A), with highest density in cortical pyramidal neurons. In rodent neurons and cortex, psychedelics increased neurite growth and dendritic spines through 5-HT2A, TrkB and mTOR signaling (Ly et al., 2018; Shao et al., 2021). A BDNF surge after psilocybin has not been shown in humans.
Functional neuroimaging studies after psilocybin show acute disintegration of the default mode network, a network associated with self-referential processing and rumination, followed by altered functional connectivity at follow-up. For the broader mechanism, see /research and /science/default-mode-network.
One mediator analysis of acute psychedelic experience and outcomes (Davis et al., 2020) found that increases in psychological flexibility, the capacity to hold difficult thoughts and feelings without immediate avoidance, statistically explained much of the link between session intensity and reductions in depression and anxiety scores at follow-up. One hypothesis for durability is experience-dependent plasticity after the session; it has been tested in mice, not established in humans. See /science/neuroplasticity.
Clinical evidence: 8 cited studies
Psilocybin with psychological support for treatment-resistant depression: an open-label feasibility study
Carhart-Harris RL, Bolstridge M, Rucker J, et al.
Lancet Psychiatry · 2016
- Design
- Open-label feasibility
- N
- 12 (treatment-resistant depression)
- Primary outcome
- QIDS at week 1 and 3 months
- Reported effect
- Mean QIDS score 11.8 points lower at week 1 and 9.2 points lower at 3 months; no serious or unexpected adverse events. No placebo group.
Psilocybin with psychological support for treatment-resistant depression: six-month follow-up
Carhart-Harris RL, Bolstridge M, Day CMJ, et al.
Psychopharmacology · 2018
- Design
- Open-label, six-month follow-up
- N
- 20 (treatment-resistant depression)
- Primary outcome
- QIDS at six months
- Reported effect
- Effect sizes remained large at six months (Cohen's d = 1.4). Open-label with no control group.
Effects of Psilocybin-Assisted Therapy on Major Depressive Disorder
Davis AK, Barrett FS, May DG, et al.
JAMA Psychiatry · 2021
- Design
- Randomized, waitlist-controlled trial
- N
- 27 randomized, 24 analysed (major depressive disorder)
- Primary outcome
- GRID-HAMD at week 4
- Reported effect
- About 71% met response criteria and 54% met remission criteria at week 4.
Trial of Psilocybin versus Escitalopram for Depression
Carhart-Harris R, Giribaldi B, Watts R, et al.
New England Journal of Medicine · 2021
- Design
- Randomized, double-blind, active-comparator (escitalopram)
- N
- 59 (moderate-to-severe MDD)
- Primary outcome
- QIDS-SR-16 at week 6
- Reported effect
- No significant difference on the primary outcome (QIDS-SR-16 -8.0 vs -6.0; p=0.17); the trial was not powered for equivalence. Secondary measures, including remission (57% vs 28%), favored psilocybin but were not corrected for multiple comparisons.
Single-Dose Psilocybin for a Treatment-Resistant Episode of Major Depression
Goodwin GM, Aaronson ST, Alvarez O, et al.
New England Journal of Medicine · 2022
- Design
- Phase IIb, randomized, double-blind, dose-finding
- N
- 233 (treatment-resistant depression)
- Primary outcome
- MADRS change at week 3
- Reported effect
- MADRS change at week 3: -12.0 (25 mg), -7.9 (10 mg), -5.4 (1 mg). 25 mg vs 1 mg: -6.6 (95% CI -10.2 to -2.9); 10 mg vs 1 mg: -2.5 (p=0.18). Adverse events occurred in 77% of participants, and suicidal ideation or behavior was seen in all dose groups.
Psychological flexibility mediates the relations between acute psychedelic effects and subjective decreases in depression and anxiety
Davis AK, Barrett FS, Griffiths RR.
Journal of Contextual Behavioral Science · 2020
- Design
- Mediation analysis (cross-sectional survey)
- N
- 985 (psilocybin and other classic psychedelic users)
- Primary outcome
- Psychological flexibility as mediator of acute experience and outcomes
- Reported effect
- Increases in psychological flexibility statistically mediated the relationship between acute experience intensity and subsequent reductions in depression and anxiety scores.
Psychedelics Promote Structural and Functional Neural Plasticity
Ly C, Greb AC, Cameron LP, et al.
Cell Reports · 2018
- Design
- Preclinical (rodent and primary cortical neuron culture)
- N
- Multiple cohorts; preclinical
- Primary outcome
- Dendritic spine density and neurite growth after a single dose
- Reported effect
- Single-dose psychedelics produced rapid increases in dendritic spine density and neuritogenesis in cortical neurons.
Psilocybin induces rapid and persistent growth of dendritic spines in frontal cortex in vivo
Shao L-X, Liao C, Gregg I, et al.
Neuron · 2021
- Design
- Preclinical (in vivo two-photon imaging in mice)
- N
- Mouse cohorts (preclinical)
- Primary outcome
- Frontal-cortex dendritic spine density at 24 hours and 1 month
- Reported effect
- In mice, a single dose raised dendritic spine density and size in frontal cortex by about 10% within 24 hours, and the change persisted at one month.
Safety profile specific to depression
- Acute suicidal ideation: published trials excluded participants with acute SI. In Goodwin 2022, suicidal ideation or behavior was seen in all dose groups, and acute risk during a session requires a trained facilitator.
- Bipolar I and II: mood-elevation risk; treated as an absolute contraindication for Bipolar I and a relative-to-absolute contraindication for Bipolar II in current screening protocols.
- SSRIs and SNRIs: consistently reported to blunt psilocybin effect; your prescriber leads any taper. See /safety/medications.
- MAOIs: absolute contraindication.
- Lithium: case reports of seizure activity and serotonin-syndrome-like presentations; treated as a relative-to-absolute contraindication; our nurse practitioner reviews each case.
- Personal or family history of psychosis: absolute contraindication. See /safety/screening.
- Cardiovascular conditions: relative contraindication; psilocybin produces transient blood pressure and heart rate elevation.
Integration considerations for depression
Anhedonia, the flattening of reward and pleasure, is one of the most stubborn features of depression. Integration in the weeks after a session emphasizes scheduled re-engagement with previously valued activities, drawing on the behavioral activation literature (Jacobson 1996, Dimidjian 2006) as a structural scaffolding. The psilocybin session may temporarily lower the avoidance gradient. In our practice, integration is the work of re-establishing daily patterns, and that is not a tested protocol.
A common pattern in the days after a session is what some participants call a “pink cloud”, early elevation followed by return toward baseline. Structured integration, including written reflection and somatic practice, is how we address this; it is our practice, not a tested protocol. Group integration can mitigate the isolation that often accompanies depression, while one-to-one integration provides space for material that is harder to disclose in group.
Restarting a regular antidepressant after a session is a decision for you and your prescriber. See /safety/medications.
Practitioner perspective
“A retreat is not depression treatment, and we don’t offer it as one. If you live with depression, screening is more careful, not less, and the conversation starts with your clinician.”
Considering psilocybin care for depression?
- Read our safety overview and medication guidance guides. Many depression medications require tapering or are absolute exclusions.
- If you are medically cleared, start with our intake conversation via /journeys/heal (free, 30 min, with a licensed facilitator). We will tell you honestly if Ceremonia is the right fit — or refer you elsewhere.
We do not accept self-bookings for clinical conditions.