Ketamine: A Clinical Reference

A clinician-facing reference covering ketamine for depression — routes, dosing, protocols, mechanism, evidence, and how ketamine fits alongside ECT, TMS, and other treatments. Written for psychiatrists, primary care clinicians, and therapists seeking a working knowledge of ketamine practice.

Overview

Ketamine occupies a unique position in the depression treatment landscape. Its antidepressant mechanism is distinct from all existing small-molecule antidepressants — glutamate/NMDA-mediated rather than monoamine-mediated — and its speed of onset (hours) is dramatically faster than anything else available. For patients with severe or treatment-resistant depression, particularly with suicidality, this combination is genuinely valuable.

At the same time, ketamine is not a clean story. The evidence base is strong for acute response but less robust for long-term durability. The psychoactive effects are uncomfortable for some patients. Addiction potential is real. Insurance coverage is uneven. And the "ketamine clinic" industry has expanded faster than quality standards, creating variable care across the country.

The short version: Sub-anesthetic ketamine (0.5–1.0 mg/kg IV, or weight-based equivalents via other routes) produces rapid antidepressant effects in 50–70% of treatment-resistant depression patients within 24 hours. Effects fade over days to weeks without continued treatment. Typical initial course: 6 infusions over 2–3 weeks, followed by tapered maintenance. Esketamine (Spravato) is the only FDA-approved formulation for depression (2019).

History & regulatory context

Ketamine was first synthesized in 1962 by Calvin Stevens at Parke-Davis as part of a search for a better anesthetic than phencyclidine (PCP). Structurally, ketamine and PCP are close cousins — ketamine adds a ketone group and an amine group (hence ket + amine). FDA approval for anesthesia followed in 1970.

The antidepressant potential wasn't seriously explored until Berman et al.'s 2000 proof-of-concept study, which demonstrated robust antidepressant effects of sub-anesthetic IV ketamine. Over the next two decades, accumulating evidence culminated in FDA approval of intranasal esketamine (Spravato) in 2019 for treatment-resistant depression and 2020 for depression with suicidal ideation.

Racemic IV ketamine remains off-label for depression but is widely used. The landscape now includes a mix of FDA-approved Spravato programs, traditional IV ketamine clinics, and emerging home-based oral ketamine programs — each with different evidence, regulatory, and quality profiles.

Indications & patient selection

FDA-approved indications (esketamine/Spravato)

  • Treatment-resistant depression (adults) — in conjunction with an oral antidepressant
  • Major depressive disorder with acute suicidal ideation or behavior (adults)

Evidence-based off-label uses (racemic ketamine)

  • Treatment-resistant major depression
  • Bipolar depression (careful selection, monitor for switch)
  • Acute suicidality
  • PTSD (emerging evidence; less robust than for depression)
  • Adjunct for severe anxiety in depressive disorders

Ideal candidates

  • Moderate-to-severe unipolar or bipolar depression with inadequate response to 2+ antidepressant trials
  • Acute depression with significant functional impairment or suicidality
  • Medically stable (good cardiovascular health, controlled blood pressure)
  • Capacity for informed consent and willingness to engage with the dissociative experience
  • Able to arrange transportation and post-treatment supervision

Contraindications

Absolute / strong relative contraindications

  • Uncontrolled hypertension (consistently >160/100)
  • Unstable cardiovascular disease, recent MI, uncontrolled arrhythmia
  • Intracranial aneurysm or recent intracranial hemorrhage
  • Active psychosis (current or primary psychotic disorder)
  • Active substance use disorder, particularly ketamine or PCP
  • Pregnancy (teratogenicity in animal models; limited human data)
  • Severe hepatic impairment
  • Hypersensitivity to ketamine

Relative contraindications / increased monitoring

  • Controlled but significant cardiovascular disease
  • History of severe dissociative or panic responses to substances
  • Bipolar I with recent manic/mixed episode
  • Severe hepatic/renal dysfunction (affects clearance)
  • Thyrotoxicosis
  • Increased intraocular or intracranial pressure
  • Benzodiazepine use (reduces efficacy; consider tapering if possible)

Medication interactions that reduce efficacy

  • Benzodiazepines: blunt antidepressant response; data suggest taper or minimize doses around infusions when feasible
  • Lamotrigine: may attenuate ketamine's effects on glutamate release
  • Naltrexone: Williams et al. 2018 suggests opioid-receptor antagonism interferes with antidepressant efficacy

Routes & formulations

Racemic ketamine structure showing R and S enantiomers

Racemic ketamine is a 50:50 mixture of R-ketamine and S-ketamine enantiomers. Spravato is pure S-ketamine (esketamine). R-ketamine has higher affinity for NMDA receptors but S-ketamine appears more potent acutely; head-to-head human trials have not directly compared them.

Off-label · Generic

IV ketamine (racemic)

The most studied route for depression. Generic; readily available through compounding pharmacies. Typical dose: 0.5–1.0 mg/kg infused over 40–60 minutes. 100% bioavailable by definition. Clinic-based administration under physician supervision (psychiatrist, anesthesiologist, or EM physician).

Pros

  • 100% bioavailability
  • Weight-based precise dosing
  • Can be discontinued immediately if adverse effects
  • Racemic mixture (may matter; unclear)
  • Strongest evidence base

Cons

  • Rarely covered by insurance
  • Requires IV placement and monitoring infrastructure
  • ~$400–800 per infusion out-of-pocket
  • Clinic-based only
FDA-approved · Esketamine

Spravato (intranasal esketamine)

Pure S-enantiomer in a nasal spray device, available in 56 mg and 84 mg dose cartridges. 35–50% bioavailability via nasal mucosa. Self-administered under clinic supervision with 2-hour post-dose observation. Only ketamine formulation with FDA approval for depression.

Pros

  • FDA-approved (MDD-TRD; MDD with suicidality)
  • Usually covered by insurance
  • No IV placement required
  • Standardized delivery
  • REMS monitoring program

Cons

  • Fixed dosing (56 or 84 mg, no weight-based adjustment)
  • Variable absorption via nasal mucosa
  • Cannot be immediately terminated mid-dose
  • 2-hour clinic observation required
  • No direct head-to-head RCT vs IV
Off-label · PO

Oral (PO) ketamine

Compounded lozenges or rapid-dissolve tablets. 20–30% bioavailability with extensive first-pass metabolism producing norketamine (which is ~33% as potent as parent compound, so oral dosing produces a different metabolite profile than IV). Emerging use in home-based telemedicine programs.

Pros

  • Home-based administration
  • Low cost
  • No procedure required

Cons

  • Variable absorption; unreliable dosing
  • Less robust evidence than IV or Spravato
  • Higher abuse potential without supervision
  • Quality/safety varies widely across telehealth programs
Off-label · IM

Intramuscular (IM) ketamine

93% bioavailability, typical dose 0.5–1.0 mg/kg IM. Limited clinical literature (Chilukuri et al. 2014 showed feasibility) but used in some settings as an alternative to IV. Faster onset than oral, less precise dose control than IV.

Protocols & dosing

Standard IV ketamine induction protocol (TRD)

Induction phase

  • Dose: 0.5 mg/kg IV over 40 minutes (may increase to 0.75–1.0 mg/kg for inadequate response)
  • Frequency: 2–3 times per week for 2–3 weeks (6 total infusions typical)
  • Monitoring: BP, HR, SpO2, CADSS (Clinician-Administered Dissociative States Scale), PACU-like recovery until ambulatory
  • Duration: Assess response weekly; 6 infusions is a reasonable trial

Maintenance phase (post-induction)

  • Taper schedule: Typically weekly → every 2 weeks → every 3–4 weeks as tolerated
  • Goal: Minimum effective frequency that maintains remission
  • Typical maintenance interval: 2–6 weeks depending on individual response

Spravato (esketamine) protocol

Treatment-resistant depression

  • Starting dose: 56 mg (day 1); may increase to 84 mg based on response/tolerability
  • Weeks 1–4 (induction): Twice weekly
  • Weeks 5–8 (transition): Once weekly
  • Week 9+ (maintenance): Once weekly or every 2 weeks at effective dose
  • Assessment: Re-evaluate therapeutic benefit at 4 weeks to determine continuation

MDD with suicidality

  • Initial dose: 84 mg (reducible to 56 mg for tolerability)
  • Weeks 1–4: Twice weekly
  • Duration beyond 4 weeks: Not evaluated in pivotal trials; continued therapy requires clinical judgment

Pharmacology

Quick-reference values from UpToDate and primary literature. Values may vary slightly by source.

Ketamine PK/PD quick reference

Onset (IV)<30 sec
Onset (IM)3–4 min
Onset (IN)~10 min
Onset (PO)~30 min
Duration (IV)5–10 min anesthetic; 1–2 hr recovery
Duration (IM)12–25 min anesthetic; 3–4 hr recovery
Half-life (alpha)10–15 min
Half-life (beta)~2.5 hr
Bioavail (IV)100%
Bioavail (IM)~93%
Bioavail (IN)35–50%
Bioavail (PO)20–30%
Vd2.1–3.1 L/kg
Protein binding~27%
MetabolismHepatic (CYP3A4, CYP2B6)
Main metaboliteNorketamine (33% parent potency)
ExcretionUrine ~91%; feces ~3%
Antidepressant dose (IV)0.5–1.0 mg/kg over 40–60 min

Mechanism of action

Ketamine's antidepressant mechanism is multifaceted and not fully resolved. Zanos and Gould (2018) proposed four complementary mechanisms, each with supporting evidence.

The basic pharmacology

Ketamine acts at three receptor types with relevance to antidepressant effects:

  • NMDA receptor — antagonist (primary mechanism of interest)
  • AMPA receptor — indirect agonist via downstream glutamate release
  • Opioid receptor — agonist; antidepressant effect is attenuated by naltrexone pre-treatment (Williams et al. 2018)

NMDA receptor background

NMDA receptors are ligand-gated, glutamatergic ion channels. They have 7 known subunits (GluN1, GluN2A-D, GluN3A-B). Typical configuration: 2 GluN1 + 2 GluN2 subunits. NMDA activation requires both glutamate binding and post-synaptic depolarization (typically from AMPA activation) to release the magnesium block and allow Ca2+ influx.

Ketamine's selectivity matters: ketamine shows higher affinity for GluN2D-containing NMDA receptors, which are preferentially expressed on forebrain inhibitory (GABAergic) interneurons. This selectivity is key to the disinhibition hypothesis.

Proposed mechanisms (Zanos 2018)

A. Disinhibition hypothesis

Ketamine blocks NMDA receptors on GABAergic interneurons → reduced inhibitory tone on pyramidal neurons → glutamate release surge → AMPA activation → BDNF release → TrkB activation → mTORC1 activation → increased synaptic protein synthesis → synaptogenesis and enhanced plasticity.

Ketamine blocking NMDA receptors on GABAergic interneurons, leading to glutamate disinhibition

The disinhibition mechanism: ketamine antagonizes NMDA receptors preferentially on GABAergic inhibitory interneurons, reducing inhibitory tone on cortical pyramidal neurons. The resulting glutamate surge activates AMPA receptors on excitatory neurons.

BDNF release and mTOR activation pathway downstream of AMPA activation

Downstream cascade following AMPA activation: BDNF release → TrkB activation → mTORC1 signaling → increased synaptic protein translation → synaptogenesis and synaptic plasticity.

B. Inhibition of extrasynaptic NMDA receptors

Ketamine antagonizes GluN2B-containing NMDA receptors located outside the post-synaptic density. These extrasynaptic receptors normally provide tonic suppression of protein synthesis machinery; blocking them disinhibits mTORC1 directly, driving the same plasticity cascade.

Ketamine blocking extrasynaptic GluN2B-containing NMDA receptors and disinhibiting mTORC1

Ketamine's antagonism of extrasynaptic GluN2B NMDA receptors releases tonic inhibition of mTORC1 signaling, providing a second route to increased synaptic protein synthesis.

C. Blockade of spontaneous NMDA activation

Spontaneous (resting-state) NMDA receptor activity normally activates eEF2 kinase, which phosphorylates eEF2 and suppresses BDNF translation. Ketamine's NMDA blockade inhibits this tonic pathway, allowing BDNF translation to proceed → TrkB activation → plasticity cascade.

D. Hydroxynorketamine (HNK) metabolite pathway

Ketamine is metabolized to (2R,6R)-HNK and (2S,6S)-HNK. In animal models (Zanos 2016), these metabolites produce antidepressant effects via direct AMPA receptor potentiation — independent of NMDA receptor action. Lepack et al. 2014 demonstrated BDNF is essential; BDNF knockouts and BDNF-neutralizing antibodies block the antidepressant response.

Ketamine metabolism to HNK and direct AMPA potentiation

Fourth proposed mechanism: ketamine's HNK metabolites directly potentiate AMPA receptors, producing antidepressant effects independent of NMDA receptor antagonism. This pathway may explain why R-ketamine (which produces more HNK) shows durable antidepressant effects in preclinical models despite lower NMDA affinity than S-ketamine.

The convergent picture

All four proposed mechanisms converge on increased synaptic protein synthesis → synaptogenesis → enhanced plasticity. This is consistent with imaging findings: Chen et al. 2018 demonstrated increased activation in the supplementary motor area and dorsal anterior cingulate cortex following 0.5 mg/kg IV ketamine in TRD patients. Multiple mechanisms likely contribute, and their relative importance probably varies across patients.

A clinically important wrinkle: Abdallah et al. 2020 found that pre-treatment with the mTORC1 inhibitor rapamycin actually prolonged ketamine's antidepressant effects (29% remission at 2 weeks vs 7% placebo), complicating the simple "mTOR is the final common pathway" narrative. The mechanism picture remains an active area of research.

Evidence base

Proof of concept: Single-dose studies

Study N Design Key finding
Berman 2000 7 RCT single-dose IV ketamine (0.5 mg/kg) vs. saline 14-point HAMD reduction in ketamine vs. 0 in control
Zarate 2006 17 RCT crossover; single-dose IV ketamine vs. placebo 71% response, 29% remission at 24h; 35% sustained at 1 week
Murrough 2013 73 RCT IV ketamine vs. active control (midazolam) 64% response at 24h vs. 28% midazolam (MADRS); median relapse at 18 days
Lapidus 2014 20 RCT crossover; intranasal ketamine 50 mg vs. saline 44% response at 24h vs. 6% placebo (MADRS)

Esketamine pivotal trials

Study N Design Key finding
Singh 2016 30 RCT esketamine 0.2 / 0.4 mg/kg IV vs. placebo MADRS reduction: 16.8 / 16.9 (active) vs. 3.8 (placebo) at 24h
TRANSFORM / SUSTAIN (pivotal) >600 Multiple Phase 3 RCTs for Spravato approval Significant improvement in TRD vs. active placebo + antidepressant; durability shown in SUSTAIN-1

Meta-analyses

McGirr 2015: Meta-analysis of RCTs confirmed single-dose ketamine efficacious for rapid treatment of unipolar and bipolar depression. Effects peaked at 24h, with residual benefit up to 7 days.
Nikayin 2023 (Lancet eClinicalMedicine): Meta-analysis comparing racemic ketamine and esketamine. Effect sizes for depression severity, response, and remission rates were numerically greater for racemic ketamine than esketamine, though direct head-to-head comparison is absent. Higher doses more effective than lower.
Price 2009: Demonstrated reduction of both explicit and implicit measures of suicidality after ketamine administration — one of the earliest studies of ketamine specifically for suicidal ideation.

Ketamine vs. ECT

The most clinically important comparative question, with a growing literature. The short answer: it depends on the presentation.

Study N Design Finding
Anand 2023 (NEJM) 365 Open-label non-inferiority; IV ketamine vs. ECT in non-psychotic TRD 55.4% response with ketamine vs. 41.2% with ECT at 3 weeks. Ketamine non-inferior to ECT.
Ekstrand 2021 (KetECT) 186 Open-label non-inferiority 63% remission with ECT vs. 46% with ketamine. ECT superior but ketamine still effective.
Rhee 2022 (JAMA Psychiatry) 340 (6 trials) Meta-analysis SMD −0.69 favoring ECT for acute symptom severity. ECT superior but patient-centered considerations matter.

Clinical synthesis

The literature is not contradictory so much as nuanced. ECT retains an edge in severe, psychotic, or catatonic presentations — Anand specifically excluded psychotic depression, and Rhee's superiority finding reflects acute symptom reduction in generally more severe populations. Ketamine is a reasonable alternative in non-psychotic TRD, particularly when patients prioritize avoiding anesthesia, cognitive side effects, or the logistical burden of an ECT course.

In practice: ECT for psychotic depression, catatonia, severe inanition, or acute suicidality with severe depression. Ketamine for non-psychotic TRD where rapid onset matters but ECT is not accepted or not feasible.

Durability & maintenance strategies

Acute antidepressant response to ketamine is reliable but short-lived. Murrough et al. 2013 found a median time to relapse of 18 days after a single infusion in TRD patients. This creates the fundamental clinical challenge: how to sustain the acute response.

Strategies with evidence

  • Continued infusions: Phillips 2019 showed that weekly continuation after an initial course maintains response significantly better than abrupt discontinuation. McMullen 2021 review concluded continued infusions are currently the most effective strategy for sustaining improvement.
  • CBT augmentation: Wilkinson 2021 randomized CBT-augmented ketamine vs. ketamine alone, finding significantly better maintenance of response at 14 weeks.
  • Behavioral activation: Phillips 2023 case series suggested behavioral activation therapy can prolong antidepressant effects; smaller evidence base but mechanistically plausible.

Rapamycin: an interesting wrinkle

Abdallah et al. 2020 pre-treated with rapamycin (mTORC1 inhibitor) before ketamine, expecting to block the antidepressant effect. Instead, rapamycin pre-treatment prolonged the response: 41% response and 29% remission at 2 weeks in the rapamycin+ketamine group vs. 13% and 7% in placebo+ketamine. The finding complicates the simple "mTOR as final common pathway" model and suggests an underappreciated role for autophagy or alternative plasticity mechanisms. Not yet a standard clinical strategy but actively researched.

Clinical takeaway: Ketamine should be framed from the beginning as part of an overall treatment plan — not a one-time intervention. Set expectations explicitly: acute response is common, durability requires continued treatment and typically psychotherapeutic augmentation.

Safety & monitoring

Acute monitoring during infusion

  • Blood pressure every 15 minutes during infusion (ketamine typically elevates BP 10–30 mmHg systolic; manage with labetalol if needed)
  • Heart rate, SpO2 continuous
  • CADSS (Clinician-Administered Dissociative States Scale) assessment during/after
  • Emergency airway equipment available
  • Dedicated staff for observation during and after infusion

Common acute side effects

  • Dissociation/altered perception (expected; target effect to some degree)
  • Hypertension, tachycardia
  • Anxiety, sometimes with panic-like quality
  • Nausea, occasionally vomiting
  • Blurred vision, dizziness
  • Headache (next-day, common)

Longer-term considerations

  • Abuse potential: ketamine is a DEA Schedule III controlled substance. Risk is managed by clinic-based administration; home-based oral protocols carry higher risk.
  • Bladder toxicity: chronic high-dose ketamine (typically recreational, far exceeding therapeutic doses) can cause ulcerative cystitis. Therapeutic-dose risk is low but monitor for urinary symptoms in long-term maintenance.
  • Cognitive effects: some longitudinal concern about chronic use; therapeutic-dose data is reassuring but not extensive.
  • Tolerance: can develop with frequent use; may necessitate dose increases or drug holidays.

Quality concerns in the ketamine clinic landscape

A practical reality: the ketamine clinic industry has expanded faster than clinical quality standards. Some programs are excellent: integrated with psychiatric care, careful screening, monitored administration, post-treatment follow-up. Others offer infusions with minimal screening or integration. When referring, due diligence on the specific program matters — not all ketamine clinics are equivalent.

Referring a patient

The Prisma Health Neuromodulation Program evaluates patients for ketamine treatment alongside other neuromodulation options (TMS, ECT) to help identify the best fit.

What to include in a referral

  • Primary diagnosis and target symptoms
  • Medication history with doses, durations, responses (simple summary is fine)
  • Prior neuromodulation (TMS, ECT) with responses
  • Cardiovascular history, current BP control, any cardiac medications
  • Substance use history
  • Benzodiazepine use (doses, indication; we may want to taper prior to treatment)
  • Suicidality assessment
  • Urgency level

Who to flag as urgent

  • Active suicidality in severe depression unresponsive to medications
  • Severe depression with functional collapse requiring rapid response
  • Post-ECT patients needing rapid bridge treatment
  • Currently inpatient requiring expedited outpatient continuation

Prisma Health Neuromodulation Program

For ketamine referrals, consultations, or clinical questions:

(864) 455-8813

Behavioral Health and Wellness Pavilion
725 Grove Road, Greenville, SC 29605

Adam Hart, MD — Medical Director, Neuromodulation Program

References

Key primary sources for ketamine in depression, organized thematically.

Proof of concept & foundational trials

  1. Berman RM, Cappiello A, Anand A, et al. Antidepressant effects of ketamine in depressed patients. Biol Psychiatry 2000;47(4):351–354. PMID: 10686270
  2. Zarate CA Jr, Singh JB, Carlson PJ, et al. A randomized trial of an N-methyl-D-aspartate antagonist in treatment-resistant major depression. Arch Gen Psychiatry 2006;63(8):856–864. PMID: 16894061
  3. Murrough JW, Iosifescu DV, Chang LC, et al. Antidepressant efficacy of ketamine in treatment-resistant major depression: a two-site randomized controlled trial. Am J Psychiatry 2013;170(10):1134–1142. PMID: 23982301
  4. Lapidus KA, Levitch CF, Perez AM, et al. A randomized controlled trial of intranasal ketamine in major depressive disorder. Biol Psychiatry 2014;76(12):970–976.
  5. Singh JB, Fedgchin M, Daly EJ, et al. A Double-Blind, Randomized, Placebo-Controlled, Dose-Frequency Study of Intravenous Ketamine in Patients With Treatment-Resistant Depression. Am J Psychiatry 2016;173(8):816–826.
  6. McGirr A, Berlim MT, Bond DJ, et al. A systematic review and meta-analysis of randomized, double-blind, placebo-controlled trials of ketamine in the rapid treatment of major depressive episodes. Psychol Med 2015;45(4):693–704.
  7. Nikayin S, Rhee TG, Cunningham ME, et al. Evaluation of the trajectory of depression severity with ketamine and esketamine treatment. Lancet eClinicalMedicine 2023;62:102130.

Mechanism of action

  1. Zanos P, Gould TD. Mechanisms of ketamine action as an antidepressant. Mol Psychiatry 2018;23(4):801–811. PMID: 29532791
  2. Murrough JW, Abdallah CG, Mathew SJ. Targeting glutamate signalling in depression: progress and prospects. Nat Rev Drug Discov 2017;16(7):472–486.
  3. Zanos P, Moaddel R, Morris PJ, et al. NMDAR inhibition-independent antidepressant actions of ketamine metabolites. Nature 2016;533(7604):481–486.
  4. Lepack AE, Fuchikami M, Dwyer JM, et al. BDNF release is required for the behavioral actions of ketamine. Int J Neuropsychopharmacol 2014;18(1):pyu033.
  5. Williams NR, Heifets BD, Blasey C, et al. Attenuation of antidepressant effects of ketamine by opioid receptor antagonism. Am J Psychiatry 2018;175(12):1205–1215.
  6. Chen MH, Li CT, Lin WC, et al. Persistent antidepressant effect of low-dose ketamine and activation in the supplementary motor area and anterior cingulate cortex in treatment-resistant depression. J Affect Disord 2018;225:709–714.
  7. Hansen KB, Yi F, Perszyk RE, et al. Structure, function, and allosteric modulation of NMDA receptors. J Gen Physiol 2018;150(8):1081–1105.
  8. Abdallah CG, Averill LA, Gueorguieva R, et al. Modulation of the antidepressant effects of ketamine by the mTORC1 inhibitor rapamycin. Neuropsychopharmacology 2020;45(6):990–997.

Durability & maintenance

  1. Phillips JL, Norris S, Talbot J, et al. Single, Repeated, and Maintenance Ketamine Infusions for Treatment-Resistant Depression: A Randomized Controlled Trial. Am J Psychiatry 2019;176(5):401–409.
  2. Wilkinson ST, Rhee TG, Joormann J, et al. Cognitive Behavioral Therapy to Sustain the Antidepressant Effects of Ketamine in Treatment-Resistant Depression: A Randomized Clinical Trial. Psychother Psychosom 2021;90(5):318–327.
  3. Phillips JL, Blier P, Talbot J. Sustaining the benefits of intravenous ketamine with behavioral activation therapy for depression: A case series. J Affect Disord Rep 2023;14:100613.
  4. McMullen EP, Lee Y, Lipsitz O, et al. Strategies to Prolong Ketamine's Efficacy in Adults with Treatment-Resistant Depression. Adv Ther 2021;38(6):2795–2820.

Comparisons with ECT

  1. Anand A, Mathew SJ, Sanacora G, et al. Ketamine versus ECT for Nonpsychotic Treatment-Resistant Major Depression. N Engl J Med 2023;388(25):2315–2325. PMID: 37224232
  2. Rhee TG, Shim SR, Forester BP, et al. Efficacy and Safety of Ketamine vs Electroconvulsive Therapy Among Patients With Major Depressive Episode: A Systematic Review and Meta-analysis. JAMA Psychiatry 2022;79(12):1162–1172.
  3. Ekstrand J, Fattah C, Persson M, et al. Racemic Ketamine as an Alternative to Electroconvulsive Therapy for Unipolar Depression: A Randomized, Open-Label, Non-Inferiority Trial (KetECT). Int J Neuropsychopharmacol 2022;25(5):339–349.
Prediction & non-response

Who responds to ketamine — and can we tell before starting?

Response rates across ketamine and esketamine studies typically land in the 50–70% range, meaning roughly 30–50% of TRD patients don't meet response criteria. Biomarker-based prediction for ketamine is less mature than for TMS or ECT — no single pretreatment variable has been robustly validated (Turrini 2024). That said, several clinical features and imaging signals carry real, replicated signal worth knowing.

Three reasons patients don't respond

Ketamine non-response, like non-response to any antidepressant, usually reflects one of three underlying factors. Distinguishing them matters because the right next move is different for each.

Biology

True non-responders

The NMDA/AMPA-mediated glutamatergic cascade that underlies ketamine's antidepressant effect doesn't translate into circuit-level change in some patients. Baseline amygdala–sgACC functional connectivity and BDNF dynamics differ between responders and non-responders in small imaging series (Nakamura 2021; Haile 2014), and genetic variation in BDNF val66met has been associated with attenuated response (Laje 2012). These signals are real but small and mostly non-actionable at the individual level today.

Medical context

The treatment lands in poor soil

Untreated thyroid disease, sleep disturbance, chronic pain, uncontrolled substance use (especially alcohol), and metabolic inflammation can all attenuate ketamine response. Patients on agents that modulate NMDA signaling (benzodiazepines at high dose, lamotrigine) may have blunted response and require discussion about weaning or acceptance of a smaller effect. Vitamin B12 status, cognitive function, and inflammatory markers each show preliminary association with response (Rong 2018).

Life circumstances

We achieve acute response; the world erodes it

Ketamine's acute effects are durable in some patients and transient in others. Active interpersonal conflict, untreated trauma, isolation, or a return to a high-stress environment often track with short inter-infusion relapse and dependence on maintenance dosing. Set and setting, integration work, and concurrent psychotherapy are not research curiosities — they are the factors most likely to turn a 72-hour response into a sustained one. A ketamine protocol without psychosocial scaffolding is rarely sufficient on its own.

Clinical predictors that actually carry weight

Reviews of ketamine predictor studies converge on a small set of clinical features that have been replicated across independent cohorts (Niciu 2014; Rong 2018; Turrini 2024). None is sufficient to deny a trial to a patient who would otherwise qualify, but each meaningfully shifts pre-infusion expectations.

CLINICAL

Family history of alcohol use disorder

Phelps 2009; Niciu 2014

First-degree relatives with alcohol dependence — a proxy for inherited glutamatergic phenotype — associated with larger and more durable antidepressant response to a single ketamine infusion. Replicated in both MDD and bipolar depression. One of the most consistent pretreatment predictors in the literature.

CLINICAL

Higher BMI

Niciu 2014; Murrough 2013

Higher BMI predicts greater acute (230-minute, day-1) improvement, though less clearly sustained at day 7+. Potentially reflects metabolic-inflammatory signaling that interacts with ketamine's mechanism. Observed across multiple cohorts, though effect size is modest.

CLINICAL

Anxious depression / anhedonia

Ionescu 2014; Lally 2014

Patients with dimensional anxious depression show greater and longer response durations; baseline anhedonia severity has been associated with greater response in at least one community cohort. Bipolar depression with baseline anhedonia showed a 73% relative increase in response likelihood in one analysis.

IMAGING

Amygdala–sgACC connectivity

Nakamura 2021

Resting-state fMRI showed differential right amygdala–subgenual ACC connectivity between ketamine responders and non-responders at baseline in TRD. Small sample (n=15), but converges with the broader sgACC-as-hub theme emerging across depression treatments. Not yet clinically actionable.

EEG / EARLY RESPONSE

Post-infusion gamma power

Turrini 2024 systematic review

The most replicated longitudinal biomarker across 69 imaging studies: post-treatment increases in frontoparietal gamma power correlate with response. Potentially a pharmacodynamic readout that could help identify likely responders after the first infusion — more useful for dose adjustment than pre-treatment selection.

BEHAVIORAL

Response to first infusion

Zhan 2019; Pennybaker 2017

Perhaps the most practical: response trajectory after a single infusion robustly predicts response to a full repeated-dosing series. Patients with meaningful early improvement tend to maintain it; non-responders at 24–48 hours rarely convert to responders with additional infusions at the same dose.

Ketamine prediction is not yet precision medicine — but it is no longer pure trial-and-error either.

Features that do not carry the weight they're often assumed to

  • Severity alone — baseline MADRS/HDRS severity is not a consistent predictor. Severely depressed patients do not uniformly respond better or worse.
  • Bipolar vs. unipolar — ketamine appears to work in both populations with broadly comparable acute response rates, though bipolar patients may require closer mood monitoring.
  • Number of prior treatment failures — unlike ECT, where episode chronicity and medication resistance predict poorer response, ketamine's effect size does not robustly track with prior treatment failure count.
  • Dissociation intensity — conflicting evidence. Some studies associate greater peri-infusion dissociation with better acute response; others don't replicate. Not useful as a selection criterion.

What this means for practice today

Ketamine biomarker-guided selection is not yet clinical reality. What is deployable today:

  • Attend to the three replicated clinical predictors — family history of alcohol use disorder, higher BMI, and anxious or anhedonic presentation — when setting pre-treatment expectations and counseling patients.
  • Treat the medical context first. Untreated sleep, thyroid, B12, substance use, and inflammatory conditions should be addressed before — or concurrent with — an infusion series rather than ignored.
  • Use the first-infusion response as a practical stratification signal. A patient with minimal response at 24–48 hours is unlikely to convert with additional standard-dose infusions; this is a point to reassess dose, frequency, protocol, or alternative treatment — not to reflexively extend the series.
  • Build the psychosocial scaffolding — integration, therapy, peer support, addressing environmental stressors — as a core element of the protocol rather than an optional add-on. Durability depends on it.
  • Be honest with patients about what's known and not known. The evidence for acute ketamine effect is strong; the evidence for individual prediction is weak; the field is moving — but not fast enough to wait on for a patient in front of you.

Patients who don't respond to ketamine aren't failing the treatment. The treatment is running into something the protocol alone can't reach — biological, medical, or psychosocial — and the clinical task is to figure out which, and adjust the plan accordingly.

Selected references for this section
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  2. Rong C, Park C, Rosenblat JD, et al. Predictors of response to ketamine in treatment resistant major depressive disorder and bipolar disorder. Int J Environ Res Public Health. 2018;15(4):771. doi:10.3390/ijerph15040771. PMID: 29673146.
  3. Phelps LE, Brutsche N, Moral JR, Luckenbaugh DA, Manji HK, Zarate CA Jr. Family history of alcohol dependence and initial antidepressant response to an N-methyl-D-aspartate antagonist. Biol Psychiatry. 2009;65(2):181–184. doi:10.1016/j.biopsych.2008.09.029. PMID: 18996507.
  4. Nakamura T, Tomita M, Horikawa N, et al. Functional connectivity between the amygdala and subgenual cingulate gyrus predicts the antidepressant effects of ketamine in patients with treatment-resistant depression. Neuropsychopharmacol Rep. 2021;41(2):168–178. doi:10.1002/npr2.12165.
  5. Ionescu DF, Luckenbaugh DA, Niciu MJ, et al. A single infusion of ketamine improves depression scores in patients with anxious bipolar depression. Bipolar Disord. 2014;16(4):394–400. doi:10.1111/bdi.12182.
  6. Turrini A, Raschellà S, Fregna L, et al. Brain-based correlates of antidepressant response to ketamine: a comprehensive systematic review of neuroimaging studies. Lancet Psychiatry. 2024;11(1):50–64. doi:10.1016/S2215-0366(23)00183-9.