Ketamine vs DXM: The Neuroscience

Put a bottle of children’s cough syrup next to an anesthesiologist’s vial of ketamine and you would never guess they are relatives. One is bought off a shelf for a few dollars; the other is drawn up under a controlled-substance license. One is associated with sniffles, the other with operating rooms and, more recently, depression clinics. And yet at the level of molecular pharmacology they belong to the same small family — both are non-competitive NMDA-receptor antagonists, both are dissociatives, and both have, against all odds, become FDA-approved ingredients in antidepressant medicines. This is a comparison of convergence: two drugs that started in opposite worlds and met at the same glutamate switch. This article is education, not medical advice.

OOTW has covered each on its own — Ketamine: The Neuroscience and DXM (Dextromethorphan): The Neuroscience. This piece sets them side by side, both to make the convergence legible and to keep the honest differences — in potency, mechanism, access, and risk — in clear view. (Educational overview only — not medical advice. If you are in crisis, contact a local emergency line or the 988 Suicide and Crisis Lifeline in the US.)

The shared premise: block the NMDA channel

For most of the twentieth century, treating depression meant one chemical story — serotonin — and the SSRIs that took weeks to nudge it. Then a different mechanism broke the plot open: blocking the NMDA glutamate receptor, without touching serotonin at all, could lift severe depression far faster. Ketamine became the flagship of that idea. What is genuinely surprising is that dextromethorphan — a molecule most people associate with a cough — shares the same headline action. Both drugs sit inside the open ion channel of the NMDA receptor and block it (uncompetitive, open-channel antagonism), the defining action of the dissociative class that also includes PCP and nitrous oxide (Taylor et al., Pharmacol Ther 2016). Hold that shared doorway in mind, because almost everything else about these two drugs diverges from it.

The comparison at a glance

The remarkable convergence: two worlds, one antidepressant mechanism

The spine of this comparison is not the shared receptor alone — it is that two molecules from opposite ends of medicine independently became NMDA-based antidepressants. Ketamine’s route ran through psychiatry’s front door. Its (S)-enantiomer, esketamine (Spravato), was FDA-approved in March 2019 for treatment-resistant depression as an add-on to an oral antidepressant, then in 2020 for depression with acute suicidal ideation, and in January 2025 as a standalone monotherapy — the first such approval for treatment-resistant depression (AJMC 2025; Johnson & Johnson 2025).

DXM arrived from the pharmacy aisle. It is the pharmacologically active half of Auvelity (AXS-05), a dextromethorphan–bupropion tablet approved in August 2022 as the first oral NMDA-receptor antagonist for major depressive disorder (Iosifescu et al., GEMINI 2022). The framing at approval was explicitly ketamine-adjacent: a rapid-acting, glutamatergic antidepressant, but delivered as a daily pill rather than a monitored infusion. That an anesthetic and a cough suppressant would converge on the same mechanism, and both clear the FDA within three years of each other, is the most striking fact in the whole comparison.

Mechanistic contrast: a potent single blow versus a soft multi-target chord

Here the two part company. Ketamine is a potent NMDA antagonist, and its prevailing antidepressant model is the disinhibition hypothesis: it preferentially silences NMDA receptors on inhibitory GABA interneurons, taking the brakes off nearby excitatory neurons and producing a brief, sharp glutamate surge in the prefrontal cortex. That surge activates a second glutamate receptor, AMPA, kicking off a cascade — BDNF release, mTOR activation — that grows new dendritic spines and synapses; block the AMPA step and the antidepressant effect vanishes, evidence that the plasticity, not the dissociation, does the work (Kim & Monteggia 2023). Two live debates complicate the neat picture. The enantiomers: racemic ketamine is a 50:50 mix of (S)- and (R)-ketamine; esketamine (the (S)-form) is the more potent NMDA blocker and the approved drug, yet (R)-ketamine shows antidepressant activity despite weaker NMDA binding. And the hydroxynorketamine (HNK) debate: a landmark study argued ketamine’s metabolite (2R,6R)-HNK produces antidepressant effects largely independent of NMDA inhibition — a claim still contested a decade later (Zanos et al., Nature 2016). NMDA antagonism is ketamine’s leading mechanism, not its settled one.

DXM plays a different, quieter game. As an NMDA antagonist it is comparatively weak — considerably less potent than ketamine — and, tellingly, much of its antidepressant signal is attributed elsewhere. Its major active metabolite, dextrorphan, is a stronger NMDA antagonist than the parent drug and drives much of the dissociative, PCP-like character at high doses (Journey & Bentley, StatPearls 2023). But DXM is a genuine multitarget molecule: a high-affinity sigma-1 receptor agonist, a serotonin–norepinephrine reuptake inhibitor (SNRI-like), and a nicotinic-acetylcholine antagonist (Stahl, CNS Spectrums 2019). Crucially, its antidepressant-like effects in animals depend substantially on sigma-1 agonism, not NMDA blockade (Nguyen et al., PLOS ONE 2014). Sigma-1 is the one target the two drugs share beyond NMDA — but even there the asymmetry is large: DXM binds sigma-1 in the nanomolar range (~138–652 nM), while ketamine’s sigma affinity is far weaker (micromolar). So where ketamine is a potent, relatively focused NMDA blow that unleashes plasticity, DXM is a softer NMDA touch wrapped in a chord of sigma-1 and monoamine effects. Same receptor family; very different music.

CYP2D6: the metabolic hinge that shapes Auvelity

One difference is so consequential it changed how DXM is formulated as a medicine. DXM is broken down to dextrorphan by cytochrome P450 2D6 (CYP2D6), one of the most genetically variable enzymes in human pharmacology. People range from poor to ultra-rapid metabolizers, and the swing is dramatic: in classic pharmacokinetic work, DXM’s half-life was roughly 29.5 hours in poor metabolizers versus about 6.6 hours in extensive metabolizers (J Clin Psychopharmacol; PMID 7593709). Extensive metabolizers clear the drug so fast it can be undetectable in plasma. To make DXM behave predictably as an antidepressant, Auvelity therefore co-formulates bupropion specifically as a CYP2D6 inhibitor, slowing the breakdown so DXM reaches and holds therapeutic brain levels across different people (the same logic behind quinidine in the older DXM medicine Nuedexta) (Stahl 2019). Ketamine has no equivalent problem: it is hepatically metabolized to norketamine and the HNK family, and is given by injection or nasal spray, bypassing the first-pass gauntlet entirely. DXM’s pharmacology is inseparable from its metabolism; ketamine’s is not.

Potency, route, and the access gap

The practical asymmetry is stark. Ketamine is delivered by IV or IM infusion (0.5 mg/kg is the classic research dose) or as the esketamine nasal spray, always in a supervised setting because of dissociation and blood-pressure effects; it is a Schedule III controlled substance available through clinics and, increasingly, telehealth. DXM is an oral, over-the-counter drug, federally unscheduled, sold in cough syrups for a few dollars. In milligram terms and in access terms they could hardly be more different: one is milligram-per-kilogram medicine administered by professionals; the other is a household product. That accessibility gap is central to both their promise and their risks — Auvelity turns the DXM mechanism into a simple daily pill, while unsupervised high-dose DXM is exactly what makes recreational misuse easy.

The experience: the K-hole versus the plateaus

Both are dissociatives, and both can detach a person from body and surroundings — but the shape of that detachment differs. Ketamine’s dissociation is dose-dependent and, at higher doses, tips into the disorienting, immobile “K-hole.” DXM’s recreational effects are described in the harm-reduction literature as four escalating “plateaus,” from mild euphoria at lower doses to near-complete, ketamine- or PCP-like dissociation at very high doses — a progression driven substantially by dextrorphan (Journey & Bentley, StatPearls 2023). This publication provides no dosing guidance; the point is simply that both drugs can produce profound dissociation, reached by an injection in one case and a bottle in the other.

The clinical scoreboard, honestly kept

Both mechanisms are now backed by real trials — for different products and at different scales. Ketamine has the larger, older evidence base. Esketamine’s monotherapy approval rested on trial data showing roughly 22.5% remission at week 4 versus 7.6% for placebo, with separation as early as 24 hours (AJMC 2025). Racemic IV ketamine, by contrast, is used widely off-label and is not FDA-approved for depression. The honest asterisk: esketamine’s approved benefit is real but modest, with persistent debate over “functional unblinding” — patients can often tell they received the active drug.

DXM’s evidence lives in Auvelity. The Phase 3 GEMINI trial (327 adults with moderate-to-severe MDD) met its primary endpoint with remission of 39.5% versus 17.3% at week 6, and the Phase 2 ASCEND trial showed superiority over bupropion alone — with statistical separation from placebo as early as week 1–2 (Iosifescu et al., GEMINI; Tabuteau et al., ASCEND 2022). The program’s record beyond depression is mixed and worth stating plainly: on April 30, 2026, the FDA expanded Auvelity to treat agitation in Alzheimer’s disease dementia — the first non-antipsychotic approved for that condition — but that approval followed a body of evidence in which one pivotal trial, ADVANCE-2, missed its primary endpoint in 2024 (FDA 2026; Axsome/BioSpace 2024). So the comparison is honest but lopsided: ketamine has the deeper depression base and a monotherapy approval; Auvelity has solid MDD data delivered as a pill, plus a newer and more uneven expansion. Neither result speaks to the other — there is no head-to-head trial comparing ketamine and DXM on any brain or clinical outcome.

The very different hazards

Neither drug is benign, and their risks differ in kind rather than degree. Ketamine’s risks concentrate in the drug itself and in chronic exposure. It carries genuine abuse and dependence liability — a concern amplified by wider telehealth access — and heavy, sustained use causes a well-documented bladder toxicity (ketamine-induced ulcerative cystitis), with urinary frequency, pain, and in severe cases irreversible damage (Ketamine, StatPearls). It transiently raises blood pressure and heart rate, and the acute dissociation, while usually brief, can be frightening without support.

DXM’s sharpest dangers come from combinations and metabolism, not the molecule alone. Because DXM inhibits serotonin reuptake, combining it with MAOIs or SSRIs can precipitate serotonin syndrome — the reason both Auvelity and Nuedexta carry explicit serotonergic warnings (case report). The most underappreciated hazard is the combination-product problem: DXM is usually sold in multi-symptom formulas, so anyone taking a large dose is also swallowing large amounts of acetaminophen (liver toxicity) or the antihistamine chlorpheniramine in “Triple C” (anticholinergic and cardiac toxicity, seizures). And CYP2D6 poor metabolizers — or people on CYP2D6-inhibiting drugs — accumulate DXM and experience amplified effects from an ordinary dose. Bupropion itself carries a boxed warning for suicidal thoughts and can lower seizure threshold, an inheritance Auvelity shares. The clean summary: ketamine’s tail risks accumulate with repeated exposure and misuse of the drug itself; DXM’s most serious harms come from serotonergic combinations, the other ingredients in the bottle, and metabolic variability. Different hazard curves, not a hierarchy.

The law in 2026

The regulatory gap mirrors the access gap. Ketamine is Schedule III — legal medicine, available through IV clinics, the approved esketamine spray, and telehealth; on December 31, 2025 the DEA and HHS extended telemedicine flexibilities keeping remote prescribing in place through the end of 2026 while permanent rules are drafted (DEA 2025). DXM remains over-the-counter and federally unscheduled; a 2010 FDA advisory committee explicitly voted against scheduling it, reasoning that it would restrict access to hundreds of legitimate cough products. The action has instead moved to the states, more than 20 of which now restrict DXM sales to buyers under 18 (CHPA 2019). None of this is legal advice — verify current federal and state law where you live.

Which one, for whom?

Resist the urge to crown a winner; the useful question is fit. For monitored, on-label treatment of treatment-resistant depression today, esketamine is the more established option, backed by the deeper trial base and a monotherapy approval — but it requires in-clinic administration and carries abuse and bladder concerns. For major depression treated as a simple daily oral pill, Auvelity delivers the NMDA-adjacent mechanism without infusions or monitoring, though its effect sizes are more modest and its story leans as much on sigma-1 and SNRI activity as on NMDA blockade. Comorbidities tilt the choice: a serotonergic medication regimen or MAOI weighs heavily against DXM; a history of substance use disorder or bladder disease weighs against ketamine. And the convergence itself is the honest headline — two drugs from opposite ends of medicine that reached the same receptor, one as a potent injected blow that unleashes plasticity, the other as a soft, metabolism-shaped, multitarget pill. It is fit, not a winner — and because no trial has ever compared them directly, the comparison is of two separate literatures that happen to rhyme.

OOTW Journal is educational and does not provide medical advice. Ketamine and dextromethorphan are potent psychoactive drugs with serious risks — ketamine carries abuse, dissociation, and bladder-toxicity concerns, and DXM can cause life-threatening serotonin syndrome when combined with SSRIs, MAOIs, or other serotonergic drugs, along with the toxicity of the other ingredients in combination cough products. Esketamine (Spravato) and Auvelity are prescription medicines that should be used only under medical supervision. This article is not a guide to using these substances. If you are struggling with depression or substance use, or in crisis, contact a clinician or the 988 Suicide and Crisis Lifeline (US). This article is education, not medical advice.