TL;DR
- Different receptor systems: psilocybin acts on 5-HT2A (serotonin), ketamine on NMDA (glutamate). Both produce rapid antidepressant effects but through distinct mechanisms.
- Ketamine has FDA approval (esketamine for TRD) and broader legal access. Psilocybin Phase 3 is in progress; Oregon and Colorado have regulated programs.
- Durability differs: psilocybin’s single dose can produce 3-6 month response in responders. Ketamine typically needs repeated infusions and tapered maintenance.
- Subjective experience differs sharply: psilocybin produces a 6-hour psychedelic experience; ketamine is dissociative for ~45 minutes per session.
- For SSRI-on patients, ketamine is often easier (no taper required for most). For psilocybin-curious patients with stable mood, the longer durability may favor psilocybin once accessible.
Side-by-side comparison
| Dimension | Psilocybin | Ketamine |
|---|---|---|
| Primary receptor | 5-HT2A partial agonist (serotonin) | NMDA antagonist (glutamate) |
| Onset of antidepressant effect | Hours to days post-session | Hours after infusion |
| Duration per dose | 3-6 months in responders (single dose can hold) | Days to ~2 weeks (single infusion) |
| Treatment course | 1-2 dosing sessions + therapy | 6-8 induction infusions over 4 weeks, then taper |
| Subjective experience | 6-hour psychedelic, visual, emotional, mystical-type | ~45 min dissociative, “out of body,” less visual |
| Approval (US, May 2026) | Phase 3 ongoing; OR/CO regulated programs; FDA expanded access rare | Esketamine (Spravato) FDA-approved for TRD; off-label IV ketamine widespread |
| Insurance coverage | Out of pocket in OR/CO ($1500-3000+/session) | Esketamine often covered; IV ketamine variable |
| SSRI compatibility | Requires 2-6 week taper for therapy | Generally compatible; many patients on SSRI during ketamine |
| Lithium compatibility | Contraindicated | Generally compatible |
| Bipolar I | Contraindicated (mania risk) | Caution; some bipolar protocols exist |
| Suicidality (rapid) | Slower onset; less data on acute suicidality | Strongest rapid-anti-suicidal evidence in TRD |
| Long-term safety | Limited long-term data; appears benign at trial doses | Bladder toxicity, dependency potential at chronic high dose |
Mechanism difference and why it matters
Psilocybin produces a brief receptor activation event (5-HT2A) followed by a 30-day window of elevated neuroplasticity (BDNF, dendritic spine density). The therapeutic logic: the dose creates a malleable state, integration therapy re-encodes depressive patterns. The single-session model depends on what happens in the weeks after.
Ketamine produces a glutamate surge through NMDA antagonism that drives rapid synaptic plasticity. The antidepressant effect comes faster but fades faster. The treatment-course logic: repeated infusions hold the new pattern in place until it can stabilize.
Patients who respond to one and not the other are common. The mechanisms are different enough that non-response to ketamine does not predict non-response to psilocybin and vice versa.
Evidence base
| Drug | Strongest data | Phase 3 status |
|---|---|---|
| Psilocybin (TRD) | Goodwin 2022 NEJM Phase 2b, n=233 | COMPASS Phase 3 in progress, reading out 2025-2026 |
| Esketamine (TRD) | FDA-approved 2019 (Spravato) based on Phase 3 program | Approved |
| Racemic IV ketamine | Multiple meta-analyses, decades of clinical use | Off-label; widely used |
| Psilocybin head-to-head with ketamine | None published | — |
Real-world access
This is where the practical decision often gets made:
- Ketamine: esketamine clinics in most major US cities and many other countries. IV ketamine clinics widespread. Insurance variable but partial coverage common. Initial induction $400-800 per infusion, lower for esketamine if covered.
- Psilocybin: Oregon and Colorado regulated programs (out-of-pocket, $1500-3000+ per session). Clinical trials at major universities (free for participants but selective). FDA expanded access exists but rare.
For most patients in 2026, ketamine is more accessible. This may shift as Phase 3 psilocybin readouts and FDA approvals progress.
When each fits
Psilocybin fits when
- You want a single-session model with longer durability per dose.
- You are interested in the experiential and integration components.
- You can taper off SSRI if needed (with prescriber).
- You are not on lithium.
- You have access (Oregon, Colorado, trial, or international).
Ketamine fits when
- You need rapid response, especially for active suicidality.
- You cannot or do not want to taper SSRI/lithium.
- You prefer a shorter session (45 min vs 6 hours).
- You want insurance coverage (esketamine often covered).
- You are willing to do repeated sessions for maintenance.
Combining them
No published protocol uses both. Some clinicians use ketamine for acute stabilization, then psilocybin for a longer-arc reset, but this is off-label sequencing rather than combination. There is no clinical trial of psilocybin + ketamine combination.
FAQ
Which is better for treatment-resistant depression?
“Better” depends on access, comorbidities, and patient preference. Both have evidence. Esketamine has FDA approval; psilocybin has stronger durability per session in the trial data we have.
Can I do ketamine while on my SSRI?
Generally yes. Most ketamine and esketamine protocols allow continued SSRI use. This is a major practical advantage over psilocybin for patients stable on SSRI.
Will ketamine help if psilocybin did not?
Possibly. Different mechanism. Some patients respond to one and not the other. No predictive biomarker yet to tell you in advance.
What about ketamine for PTSD or anxiety?
Less evidence than for depression but used off-label. For PTSD, MDMA-assisted therapy has the strongest evidence (see our PTSD page). Ketamine is being studied.
Is ketamine "psychedelic"?
It is a dissociative, not a classical psychedelic. The subjective experience differs (less visual, more disembodied) and the receptor target is glutamatergic, not serotonergic.
What about microdosing? Microdose psilocybin vs at-home ketamine lozenges?
Different evidence bases. Microdose psilocybin for depression has weak controlled-trial evidence. At-home ketamine lozenges (compounded) have grown in popularity but lack RCT evidence; bladder toxicity and dependency are real concerns at chronic doses.
