What Is Treatment-Resistant Depression?
Treatment-resistant depression (TRD) is clinically defined as major depressive disorder that has not responded to at least two adequate trials of antidepressant medication — meaning the right dose, taken for a sufficient duration (typically 6–8 weeks).
It is more common than most people realize. Approximately 30% of people with major depression do not achieve remission with standard antidepressants. For these patients, the cycle of trying medication after medication — with side effects, partial responses, and eventual discontinuation — is exhausting and demoralizing.
Important distinction:
Not all depression that hasn't responded to one medication is truly 'treatment-resistant.' Misdiagnosis (e.g., undetected bipolar disorder), inadequate dosing, or incomplete trials are common reasons for apparent treatment failure. A thorough psychiatric evaluation is essential.
How Common Is It in Israel?
Depression is the leading cause of disability worldwide, and Israel is no exception. An estimated 8–12% of the Israeli population will experience a major depressive episode in their lifetime. Among those, a significant proportion will not find adequate relief through standard psychiatric treatment pathways.
~30%
of depression patients don't adequately respond to SSRIs
2+
failed antidepressant trials required for a TRD diagnosis
60–75%
response rate to ketamine in TRD patients (clinical studies)
Why Antidepressants Sometimes Fail
SSRIs (selective serotonin reuptake inhibitors) and SNRIs work by increasing serotonin or norepinephrine availability in the brain. This mechanism helps many people — but depression is not a single disorder with a single cause.
Serotonin hypothesis limitations
The idea that depression is simply 'low serotonin' is an oversimplification. Depression involves complex disruptions across multiple neurotransmitter systems, stress hormones, inflammation, and neural circuit function.
Genetic variability
Pharmacogenomic differences mean some people metabolize antidepressants unusually fast or slow, reducing effectiveness. Others have receptor variations that diminish response.
Neural atrophy
Chronic depression is associated with shrinkage in the hippocampus and prefrontal cortex — regions involved in mood regulation and memory. SSRIs may not adequately reverse this atrophy in all patients.
Inflammatory depression
A subset of depression is driven by chronic inflammation. SSRIs have minimal anti-inflammatory effects and may be ineffective in this subtype.
Neuroplasticity: the brain's ability to form new connections
Neuroplasticity: Why Ketamine Works Differently
Ketamine acts primarily on the NMDA (N-methyl-D-aspartate) receptor — a glutamate receptor. Glutamate is the brain's primary excitatory neurotransmitter, involved in learning, memory, and synaptic plasticity.
When ketamine blocks NMDA receptors, it triggers a cascade of effects:
- 1
Rapid AMPA receptor activation increases glutamate signaling.
- 2
BDNF (brain-derived neurotrophic factor) is released — a molecule critical for neuronal growth and repair.
- 3
mTOR pathway activation promotes new synaptic connections (synaptogenesis) within hours.
- 4
The prefrontal cortex and hippocampus — atrophied in depression — begin to recover structural density.
This neuroplasticity window — most open in the hours to days following a ketamine session — is precisely when integration psychotherapy can produce lasting behavioral and emotional change. This is why KAP combines ketamine with structured therapy rather than delivering it in isolation. For a visual exploration of how ketamine affects the brain's neural circuits, see our Visual Neuroscience Library.
The Clinical Evidence for Ketamine in TRD
Ketamine has been studied for treatment-resistant depression since the early 2000s. The evidence base is now substantial:
Rapid antidepressant effects
Multiple randomized controlled trials demonstrate significant antidepressant effects within 24 hours — something no conventional antidepressant achieves.
Response rates of 60–75%
In patients who have failed multiple antidepressants, ketamine produces clinically meaningful improvement in the majority of cases.
Anti-suicidal effects
Ketamine shows rapid reduction in suicidal ideation — an effect that is particularly important in acute crisis situations and distinct from its antidepressant effect.
PTSD evidence
Emerging evidence supports ketamine for PTSD, with studies showing significant reduction in PTSD symptom severity after a small number of sessions.
FDA-approved derivative
The FDA approval of esketamine (Spravato) for TRD in 2019 validated the ketamine mechanism, lending further institutional credibility to the treatment class.
Related Reading
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