Glutamate Pathway Treatments Reshaping TRD Care

The Science Behind Why Ketamine Works When Antidepressants Don't

A detailed clinical review published in Psychiatric Times in April 2026 has brought renewed attention to glutamate-based treatments for treatment-resistant depression (TRD) — a condition affecting an estimated 30% of people diagnosed with major depressive disorder who fail to respond to two or more standard antidepressant trials. The piece centers on how targeting the glutamatergic system, rather than the traditional monoamine pathways (serotonin, norepinephrine, dopamine), can produce rapid and meaningful relief where conventional medications have failed.

The two primary agents discussed are esketamine (the nasal spray formulation marketed as Spravato) and dextromethorphan-bupropion (Auvelity), both of which act on NMDA receptors — the same receptor class that ketamine, including oral ketamine tablets, engages. The review underscores how these drugs promote synaptogenesis, the formation of new synaptic connections in the brain, which is now understood to be a core mechanism behind their antidepressant effect. This neuroplasticity-driven action is fundamentally different from how SSRIs or SNRIs work, and it helps explain why glutamate-targeting therapies can work within hours or days rather than weeks.

Where Oral Ketamine Tablets Fit in This Picture

The clinical conversation around glutamatergic depression treatment is often dominated by esketamine nasal spray and IV ketamine infusions, but oral ketamine tablets represent a growing and increasingly validated option in this same mechanistic family. Understanding how they compare is essential for patients navigating their options.

Bioavailability and absorption: Oral ketamine tablets have a bioavailability of roughly 20–30%, compared to approximately 45–50% for intranasal esketamine and close to 100% for IV infusion. This means oral dosing requires careful calibration — but it also means the route is gentler, slower to peak, and more forgiving in terms of dissociative side effects for many patients. When absorbed through the gastrointestinal tract, ketamine is partially converted to norketamine in the liver, a metabolite that itself has antidepressant properties and a longer half-life than the parent compound.

Tablets vs. troches: Oral ketamine is available in two primary forms — sublingual troches (lozenges held under the tongue) and swallowed tablets. Troches bypass first-pass metabolism to a greater degree, producing faster onset and somewhat higher bioavailability. Tablets swallowed whole move through the gut and liver first, resulting in a more delayed, smoother onset. For patients who find the dissociative effects of troches uncomfortable or unpredictable, tablets can offer a more manageable experience — though clinical response data comparing the two formats head-to-head remains limited.

Dosing considerations for TRD: Because TRD by definition involves prior treatment failure, patients in this category often require close monitoring and titration. Oral ketamine prescribed off-label for TRD is typically dosed in the range of 1–3 mg/kg, taken at home under a structured protocol guided by a prescribing clinician. The flexibility of the tablet format — no clinic visit required, no IV line, no waiting room — makes it particularly relevant for patients with access barriers, mobility issues, or high anxiety around clinical settings.

Access, Safety, and the Expanding Glutamate Landscape

One of the most important takeaways from the Psychiatric Times review is the implicit acknowledgment that the glutamatergic treatment space is no longer a niche. With FDA-approved esketamine and dextromethorphan-bupropion now part of mainstream psychiatric practice, and with growing off-label use of oral racemic ketamine, clinicians are being asked to think more precisely about which glutamate-targeting agent fits which patient.

For patients considering or currently using oral ketamine tablets, a few practical points emerge from this research context:

• Monitoring matters: Unlike IV infusions administered in a clinical setting with real-time oversight, oral ketamine taken at home requires robust telehealth check-ins, clear protocols for managing dissociation, and ideally a trusted person present during dosing sessions. The convenience advantage of tablets comes with a corresponding responsibility for patient education.
• Response timelines vary: The rapid antidepressant effects described in the glutamate research — sometimes within hours for IV ketamine — are somewhat attenuated with oral tablets due to slower absorption. Patients should expect onset over several hours rather than minutes, and sustained benefit typically requires a multi-week protocol rather than a single dose.
• Combination considerations: The dextromethorphan-bupropion data in this review is a reminder that glutamate-targeting compounds are increasingly being combined with other agents. Patients on oral ketamine should keep their prescribing clinician fully informed of all medications, as interactions — particularly with CNS depressants, MAOIs, and stimulants — can alter both safety and efficacy profiles.
• Insurance and cost: Esketamine carries FDA approval and, in many cases, insurance coverage with prior authorization. Oral ketamine tablets, prescribed off-label, are typically out-of-pocket expenses. For patients with TRD who have exhausted covered options, this cost differential is a real access barrier worth discussing openly with care teams.

The broader scientific validation of the glutamate pathway as a legitimate and effective target for depression is good news for everyone in the ketamine treatment space. It reinforces the mechanistic rationale for oral ketamine's use, supports ongoing research, and increases the likelihood that payers and regulators will eventually take a closer look at oral formats specifically. Read the full review at Psychiatric Times.