Ketamine’s ability to lift severe depression within hours rather than weeks has reshaped how researchers think about antidepressants, and a growing number of drugs now aim to replicate that speed through overlapping brain pathways. The most established is esketamine, a nasal spray derived directly from ketamine and approved by the FDA for treatment-resistant depression. But the pipeline extends well beyond ketamine’s mirror image: dextromethorphan-bupropion targets the same glutamate receptor, neurosteroids work through an entirely different system to achieve comparable speed, and psychedelics like psilocybin appear to converge on the same downstream brain-rewiring machinery. The landscape is broader and stranger than most people realize.
Why Ketamine Works Differently From Traditional Antidepressants
Standard antidepressants like SSRIs raise serotonin levels gradually, and most people wait four to six weeks before feeling meaningful relief. Ketamine sidesteps that timeline by blocking a glutamate receptor called NMDA, which triggers a cascade of rapid changes in brain signaling. Within hours, the brain ramps up production of growth factors like BDNF (a protein that supports the health and formation of new connections between neurons) and begins sprouting new synaptic connections, particularly in the prefrontal cortex and hippocampus.1PubMed Central. Neurotrophic mechanisms underlying the rapid and sustained antidepressant actions of ketamine That burst of new connectivity is thought to be the core reason ketamine works fast. It is also the template that newer drugs are trying to copy, whether they block the same receptor ketamine does or reach the same endpoint through a different door.
What makes this mechanism especially interesting is that NMDA receptor blockade alone may not be the full story. Ketamine also boosts activity at another glutamate receptor called AMPA, and it activates signaling pathways involved in building new synapses.2Europe PMC. Antidepressant effects of ketamine and the roles of AMPA glutamate receptors and other mechanisms beyond NMDA receptor antagonism Drugs that mimic ketamine do not all hit every one of these targets, which is part of why some work better than others and why side-effect profiles differ.
Esketamine, the FDA-Approved Derivative
Ketamine is a mixture of two mirror-image molecules, called the S and R forms. Esketamine is the isolated S form, approved in 2019 as a nasal spray (brand name Spravato) for treatment-resistant depression and, later, for major depression with suicidal thinking. It binds more tightly to the NMDA receptor than the R form does, which was the rationale for developing it as a standalone drug.
Whether esketamine actually works as well as the original racemic (mixed) ketamine has been a source of genuine debate. A meta-analysis pooling 24 trials and nearly 1,900 participants found that intravenous racemic ketamine had higher response and remission rates than intranasal esketamine.3PubMed Central. Comparative efficacy of racemic ketamine and esketamine for depression: a systematic review and meta-analysis A separate network meta-analysis comparing ketamine, esketamine, and common augmentation strategies like aripiprazole and lithium reached a similar conclusion: intravenous racemic ketamine was significantly more effective and better tolerated than intranasal esketamine.4PubMed. Comparative efficacy, tolerability and acceptability of intravenous racemic ketamine with intranasal esketamine, aripiprazole and lithium as augmentative treatments for treatment-resistant unipolar depression: A systematic review and network meta-analysis These comparisons are somewhat complicated by route of administration (IV versus nasal spray) and dosing differences, but the pattern has raised questions about whether isolating the S form was the right approach.
Esketamine does hold a practical advantage: it is FDA-approved and covered by many insurance plans, which changes the math for patients. A cost-effectiveness analysis found that from a strict healthcare-system perspective, esketamine was unlikely to be cost-effective compared to IV ketamine. But from the patient’s perspective, insurance coverage made esketamine similar in effectiveness and less costly out of pocket.5PubMed. Cost-effectiveness of esketamine nasal spray compared to intravenous ketamine for patients with treatment-resistant depression in the US utilizing clinical trial efficacy and real-world effectiveness estimates For many people with treatment-resistant depression, the question is not which form of ketamine is theoretically superior but which one they can actually get.
R-Ketamine and the Metabolite That May Matter More
If esketamine is the S-enantiomer story, the R-enantiomer story is arguably more surprising. In animal studies, R-ketamine produces stronger and longer-lasting antidepressant effects than S-ketamine despite binding less tightly to the NMDA receptor. It also causes fewer dissociative and psychosis-like side effects in rodents, monkeys, and humans.6PubMed. Molecular mechanisms of the rapid-acting and long-lasting antidepressant actions of (R)-ketamine That finding was counterintuitive enough to shift thinking about what really drives ketamine’s antidepressant effect: if the weaker NMDA blocker works better as an antidepressant, maybe NMDA blockade is not the main event.
One leading explanation centers on a metabolite called (2R,6R)-hydroxynorketamine, or HNK. When your body breaks down ketamine, HNK is one of the byproducts, and it appears to have antidepressant activity of its own.7PubMed Central. Mechanisms of ketamine action as an antidepressant In mouse studies, HNK produced antidepressant-like behavioral effects at doses far too low to block NMDA receptors. The concentration needed to actually inhibit NMDA receptors was roughly 35 times higher than the concentration that produced antidepressant effects, which strongly suggests HNK is working through a completely different mechanism.8PubMed Central. Antidepressant-relevant concentrations of the ketamine metabolite (2R,6R)-hydroxynorketamine do not block NMDA receptor function Some of that mechanism appears to involve a type of glutamate receptor called mGlu2, which we will return to shortly. HNK is now being explored as a potential standalone antidepressant that might deliver ketamine-like benefits without the dissociative high or abuse concerns.
Dextromethorphan-Bupropion, the First Oral Option
For decades, dextromethorphan was just a cough suppressant. But it also blocks NMDA receptors and activates sigma-1 receptors, both of which are implicated in mood regulation. The problem was that the body metabolizes dextromethorphan too quickly for it to reach therapeutic brain levels. Pairing it with bupropion, which inhibits the liver enzyme responsible for that rapid breakdown, solved the pharmacokinetic puzzle. The combination (brand name Auvelity) was approved by the FDA in 2022 for major depressive disorder, making it the first oral rapid-acting antidepressant to reach the market.
In clinical trials, patients taking dextromethorphan-bupropion saw meaningful improvement on depression scales within one to two weeks, much faster than conventional antidepressants typically deliver.9PubMed Central. Dextromethorphan-bupropion (Auvelity) for the Treatment of Major Depressive Disorder A systematic review of the trial data found that depression severity dropped significantly within one week compared to placebo, and that improvement was maintained for up to twelve months with continued use.10PubMed. Dextromethorphan-Bupropion for the Treatment of Depression: A Systematic Review of Efficacy and Safety in Clinical Trials It is not as fast as IV ketamine, which can work within hours, but the convenience of a daily pill with no clinic visit is a significant practical advantage. It also lacks ketamine’s dissociative effects and has no known abuse potential at the doses used, which matters for long-term treatment.
Neurosteroids and the GABA Route
Not all rapid-acting antidepressants target glutamate. Brexanolone (brand name Zulresso) and zuranolone (brand name Zurzuvae) are neurosteroids that work by boosting the activity of GABA-A receptors, which are the brain’s primary inhibitory system. They were developed specifically for postpartum depression, where brexanolone (an IV infusion) became the first FDA-approved treatment in 2019, and zuranolone (an oral pill) followed in 2023. Both can produce antidepressant effects within days.11PubMed Central. Rapid-acting NMDA and GABAergic Modulators in Mood Disorders: From Synaptic Mechanisms to Clinical Practice
Despite working through a different neurotransmitter system, neurosteroids and ketamine-like drugs are sometimes grouped together as “rapid-acting antidepressants” because they share the defining feature of speed. The mechanisms are genuinely distinct, though. Ketamine and its relatives boost glutamate signaling and trigger the growth of new synaptic connections from the excitatory side. Neurosteroids modulate inhibitory tone by fine-tuning GABA circuits that have gone awry, particularly in the hormonal upheaval of the postpartum period. Other neurosteroid compounds targeting GABA-A receptors have been tested for broader major depression as well. One such compound, PRAX-114, showed reductions of 15 to 19 points on a standard depression rating scale over eight days in a phase 2 study, though larger controlled trials are still needed.12Translational Psychiatry. Understanding the mechanism of action and clinical effects of neuroactive steroids and GABAergic compounds in major depressive disorder
Psychedelics and the Convergence on Neuroplasticity
Psilocybin, the active compound in “magic mushrooms,” works primarily through serotonin 2A receptors, which is a completely different entry point from ketamine’s NMDA blockade. Yet in animal models of depression, a single dose of either ketamine or psilocybin reversed depressive behaviors within 24 hours, and those effects lasted for at least two weeks. A standard SSRI (fluoxetine, better known as Prozac) required extended daily dosing to achieve comparable results.13PubMed Central. A comparative assessment of the antidepressant efficacy of ketamine, psilocybin, and fluoxetine in a chronic stress model
The reason these two chemically unrelated compounds produce such similar outcomes likely comes down to a shared downstream effect: neuroplasticity. Both ketamine and classical psychedelics increase glutamate release, activate AMPA receptors, boost BDNF signaling, and stimulate the growth of new synaptic connections in prefrontal cortex pyramidal neurons.14Trends in Pharmacological Sciences. Neuroplasticity as a convergent mechanism for ketamine and classical psychedelics A molecular study reinforced this convergence by examining gene expression changes caused by ketamine, HNK, psilocybin, and another psychedelic in human neurons. The overlap was striking: roughly 86% of the proteins altered by ketamine-class drugs were also altered by at least one serotonergic psychedelic.15Molecular Psychiatry. Convergent neuroimmune signaling underlying rapid antidepressant response to ketamine and psychedelics The starting receptors differ, but the cellular renovation project looks remarkably similar.
Psilocybin is not yet FDA-approved for depression, though large phase 3 trials are underway. If approved, it would join ketamine and esketamine in the rapid-acting category but would offer something distinct: a treatment model built around one or a few guided sessions rather than repeated dosing.
The Opioid System Wrinkle
One of the more uncomfortable findings in this field is that ketamine’s antidepressant effect appears to require the brain’s opioid system to be functional. In a clinical trial, researchers gave depressed patients ketamine alongside naltrexone, a drug that blocks opioid receptors. Naltrexone dramatically reduced ketamine’s antidepressant effect while leaving its dissociative effects completely intact, a result so striking that the trial was halted early.16PubMed Central. Opioid Receptor Antagonism Attenuates Antidepressant Effects of Ketamine Animal studies have replicated this: blocking opioid receptors with naltrexone wiped out ketamine’s antidepressant behavioral effects across multiple tests measuring despair, pleasure-seeking, and anxiety.17Translational Psychiatry. The endogenous opioid system in the medial prefrontal cortex mediates ketamine’s antidepressant-like actions
Before anyone panics about ketamine being “just an opiate,” the picture is more nuanced. Activating opioid receptors on their own does not replicate ketamine’s antidepressant effects, and ketamine does not produce the reward-seeking behavior associated with opioid drugs. Rodent studies concluded that the opioid system is “necessary but not sufficient” for ketamine’s action, meaning opioid signaling plays a permissive role rather than driving the effect directly.18PubMed Central. Opioid system is necessary but not sufficient for antidepressive actions of ketamine in rodents This distinction matters enormously for drug development. Any ketamine-like drug that manages to bypass opioid involvement entirely would be more attractive from a safety and regulatory standpoint, and it is one reason researchers are especially interested in compounds like HNK and the mGlu2/3 antagonists discussed below.
Scopolamine and the Muscarinic Pathway
Scopolamine is best known as a motion-sickness patch, but clinical studies have shown it produces rapid antidepressant effects when given intravenously to depressed patients. It works by blocking muscarinic acetylcholine receptors, which is a different receptor system from anything ketamine touches directly. Yet preclinical work has revealed that scopolamine’s antidepressant action requires glutamate receptor activation and engages the same mTOR signaling pathway that ketamine uses to build new synapses.19PubMed Central. Rapid antidepressant actions of scopolamine: Role of medial prefrontal cortex and M1-subtype muscarinic acetylcholine receptors In other words, scopolamine enters through the acetylcholine door but ends up in the same room as ketamine. The practical challenge is that IV scopolamine is not a convenient treatment, and oral versions have not shown the same antidepressant potency. But the finding reinforced a broader lesson: multiple entry points into the glutamate-BDNF-mTOR cascade can produce ketamine-like speed.
Drugs Still in the Pipeline
Several compound classes are in preclinical or early clinical development, targeting pieces of the glutamate system that ketamine affects indirectly.
mGlu2/3 Receptor Antagonists
Group II metabotropic glutamate receptors (mGlu2 and mGlu3) act as brakes on glutamate release. Blocking them with antagonists lifts that brake, flooding the prefrontal cortex with glutamate in a pattern that closely resembles what ketamine does. In animal models, mGlu2/3 antagonists produced rapid antidepressant effects and worked in stress models that do not respond to conventional antidepressants.20PubMed. mGlu2/3 receptor as a novel target for rapid acting antidepressants More targeted tools called negative allosteric modulators, which selectively dampen either mGlu2 or mGlu3, have reversed anhedonia and passive coping in chronically stressed mice through related but distinct mechanisms.21PubMed Central. mGlu(2) and mGlu(3) Negative Allosteric Modulators Divergently Enhance Thalamocortical Transmission and Exert Rapid Antidepressant-like Effects Intriguingly, ketamine’s own metabolite HNK appears to work in part through mGlu2 receptors; combining low doses of HNK with an mGlu2/3 antagonist produced synergistic antidepressant effects in mice.22PubMed Central. (2R,6R)-hydroxynorketamine exerts mGlu(2) receptor-dependent antidepressant actions No mGlu2/3 antagonist has reached late-stage clinical trials for depression yet, but the preclinical profile is strong.
AMPA Receptor Positive Modulators
Since AMPA receptor activation appears to be a critical step in ketamine’s downstream signaling, drugs that directly boost AMPA receptor function have attracted interest. Compounds called ampakines have shown rapid antidepressant-like effects in rodent models, with one compound (CX717) producing effects within 30 minutes.23PubMed. Antidepressant-Like Effects of CX717, a Positive Allosteric Modulator of AMPA Receptors However, progress has been slow. Few AMPA modulators have advanced past early clinical development, hampered by challenges like low potency and the difficulty of targeting AMPA receptors with enough specificity to get antidepressant effects without triggering seizures or excitotoxicity.24PubMed Central. Positive AMPA receptor modulation in the treatment of neuropsychiatric disorders: A long and winding road
How Ketamine Changes Brain Connectivity
Beyond the molecular level, ketamine produces measurable changes in how brain regions communicate. In people with major depression, a brain network called the default mode network often shows abnormal connectivity with the insula, a region involved in emotional awareness. A neuroimaging study found that a single ketamine infusion normalized this insula-to-default-mode-network connectivity in depressed patients within two days. The effect was specific to the ketamine condition and had reversed by ten days after treatment.25PubMed Central. Default Mode Connectivity in Major Depressive Disorder Measured Up to 10 Days After Ketamine Administration These circuit-level changes help explain both why ketamine works fast and why the effects are temporary without repeated dosing. Researchers are exploring whether brain-wave patterns like gamma oscillations might serve as biomarkers to predict who will respond to ketamine-like drugs, which could eventually help clinicians match patients to the right rapid-acting treatment.26PubMed Central. Electrophysiological biomarkers of antidepressant response to ketamine in treatment-resistant depression: Gamma power and long-term potentiation
The Neuroinflammation Connection
Depression is increasingly understood to involve brain inflammation, and ketamine may fight that too. In rats subjected to chronic stress, ketamine treatment reversed the elevated levels of inflammatory molecules (TNF-α, IL-1β, and IL-6) in the hippocampus and reduced the activation of microglia, the brain’s resident immune cells.27Neuropsychiatry. The effect of ketamine on microglia and proinflammatory cytokines in the hippocampus of depression-like rat Interestingly, the two enantiomers may differ here as well. Research on S-ketamine found that it significantly reduced inflammatory factors in the prefrontal cortex through a specific enzyme pathway, an effect that R-ketamine did not share.28PubMed Central. S-ketamine Alleviates Neuroinflammation and Attenuates Lipopolysaccharide-Induced Depression Via Targeting SIRT2 This anti-inflammatory action could be especially relevant for the subset of depressed patients who show elevated inflammatory markers, a group that tends to respond poorly to conventional antidepressants. Whether future ketamine-like drugs can be optimized to target inflammation alongside glutamate signaling is an active area of investigation.
Combining Ketamine With Psychotherapy
One emerging approach treats ketamine not as a standalone fix but as a catalyst for psychotherapy. The idea is straightforward: if ketamine opens a window of heightened neuroplasticity, doing therapy during that window might help rewire dysfunctional thought patterns more effectively. A small study combining oral ketamine with a structured psychotherapy protocol saw patients’ depression scores drop by roughly 47% on average, a clinically meaningful improvement.29European Journal of Medical and Health Sciences. Efficacy of Oral Ketamine Combined with Psychotherapy for Treatment Resistant Depression The evidence base here is still thin, limited mostly to small trials and case series. But the logic appeals to clinicians who worry about the durability of ketamine’s effects: if the drug alone produces changes that fade within a week or two, pairing it with therapy might help those changes stick. This is another area where the broader class of rapid-acting antidepressants, including psilocybin, may eventually find their most useful clinical niche.
Urological and Safety Considerations Across the Class
Ketamine’s most concerning long-term side effect in recreational users has been bladder damage, sometimes severe enough to require surgery. How much this applies to the lower, medically supervised doses used for depression is still being studied, but it is a real concern for any patient considering repeated ketamine infusions over months or years. A pharmacovigilance analysis comparing esketamine and racemic ketamine found that ketamine had higher odds of renal and urinary problems compared to other drug classes, while esketamine had lower or comparable odds.30PubMed. Comparative safety of prescribed Esketamine and ketamine in relation to renal and urinary disorders: A pharmacovigilance perspective This difference could reflect dose, route, or the specific pharmacology of the S-enantiomer, but it is one safety factor worth tracking as these treatments move into longer-term use. The newer oral options like dextromethorphan-bupropion sidestep this concern entirely since they do not involve ketamine at all. And if compounds like HNK or mGlu2/3 antagonists eventually reach the clinic, their safety profiles may look quite different from ketamine’s, given their distinct receptor targets and lack of dissociative effects.