Ketamine occupies a defined but complicated place in the refractory depression toolkit. The NMDA-antagonist mechanism is mechanistically distinct from monoaminergic antidepressants; the rapid antidepressant effect (within hours to days, versus weeks) addresses an acute clinical need that conventional pharmacology cannot meet; and the evidence base in TRD is now substantial. The complications are real: durability is limited without ongoing treatment, access and cost are uneven across delivery routes, the long-term cognitive question remains open, and the clinical infrastructure required for safe administration is non-trivial. The longevity-psychiatry frame engages ketamine as a real tool with real costs, used selectively and monitored carefully.
The mechanism is glutamatergic at the proximate level and broader downstream. Ketamine antagonizes NMDA receptors preferentially on GABAergic interneurons, producing a transient glutamate surge in the prefrontal cortex that drives BDNF release, mTOR signaling, and rapid synaptogenesis. The result is a rapid antidepressant effect that does not depend on monoaminergic pathways. The metabolite hydroxynorketamine appears to contribute substantial antidepressant effect, and the picture has expanded from simple NMDA-blockade to a more complex cascade involving AMPA receptor activation, GSK-3 inhibition, and downstream plasticity. The mechanistic novelty is real and matters clinically — patients who fail multiple monoaminergic agents may respond to ketamine because the pathway is fundamentally different.
The delivery routes do not interchange cleanly. IV racemic ketamine (0.5 mg/kg over 40 minutes) is the most evidence-based protocol; six infusions over 2–3 weeks is the standard induction; maintenance varies from monthly to less frequent. Intranasal esketamine (Spravato) is the only FDA-approved version, with twice-weekly induction phase tapering to weekly then biweekly maintenance; REMS-required administration in monitored setting; clear efficacy data but higher per-dose cost and the burden of in-office administration. Oral and sublingual compounded ketamine has emerged as a lower-cost option with less evidence; bioavailability is highly variable (approximately 20–30%); the practice is less standardized. IM ketamine is used in some clinics; less evidence than IV. The route choice depends on access, cost, severity, and the supporting infrastructure of the prescribing setting.
The durability problem is the central clinical limitation. Most patients who respond to acute ketamine relapse within weeks to months without ongoing treatment. Maintenance protocols (monthly or more frequent infusions; weekly or biweekly esketamine) extend benefit but produce treatment indefinitely rather than achieving durable monotherapy. The clinical implication is that ketamine is often a bridge or augmentation rather than a stand-alone treatment; the patient who responds to ketamine should be using the window to optimize other treatment, build behavioral foundation, address contributing factors, and progress toward a regimen that may not require indefinite ketamine maintenance.
The cognitive question is the longevity-psychiatry concern that does not have a complete answer. Chronic recreational ketamine use produces cognitive impairment, urological pathology (ketamine cystitis), and dependence; chronic therapeutic-dose ketamine has substantially better safety profile but the long-term data extend less than a decade for most patients on indefinite maintenance protocols. The cautious clinical posture is: use ketamine where it is the right answer, monitor for cognitive complaints and urological symptoms, prefer time-limited treatment with bridge-to-other-strategies over indefinite maintenance, and remain alert to emerging long-term data. The patient on ketamine maintenance for years is the population the field needs to follow carefully.
The clinical workflow for ketamine candidacy includes appropriate selection, safety screening, and integration with other treatment. Candidates are TRD patients who have failed adequate trials of layered conventional treatment; the workup excludes contraindications (uncontrolled hypertension, severe cardiac disease, active psychosis, active substance use disorder with relevant agents, pregnancy). Administration requires monitored setting (esketamine is REMS-required; IV protocols similar). Concurrent treatment continues — ketamine is augmentation, not substitution. The post-acute plan addresses what happens after the induction series: maintenance schedule, integration with other treatments, plan for tapering or transitioning off. The discipline is to deploy ketamine where it has real clinical place — rapid response in severe TRD with adequate alternatives explored — and to integrate it into a broader treatment strategy that does not depend on it indefinitely.