Here’s How Ketamine Affects The Brain

• By Sam Woolfe
• Medically approved by Abid Nazeer
• July 9, 2021June 30, 2026
• Ketamine Articles ketamine therapy psychedelics psychedelics side effects
• With 0 comments

Last reviewed and updated: June 30, 2026.

Key Takeaways

To understand how ketamine works, you need to know how it affects the brain. The therapeutic effects of ketamine come down to these brain changes, which are diverse and multi leveled.

This post takes a look at how ketamine causes changes in the brain at the level of neurotransmitters, as well as brain regions. We will also describe the psychological effects associated with these changes, such as psychedelic experiences and improvements in depression.

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Ketamine and Glutamate

At the level of neurotransmitters, we find that ketamine changes the levels of glutamate. Glutamate is an excitatory neurotransmitter. It is in a balanced state, or “homeostasis,” with GABA (gamma-Aminobutyric acid,) which is an inhibitory neurotransmitter. Think of Glutamate as the “gas pedal” and GABA as the “brakes” of central nervous system. Many factors can alter the balance in the glutamate and GABA systems, including states of high anxiety, mood changes, and physical dependence on heavy CNS depressants (e.g., benzodiazepines, alcohol, opioids). Ketamine can help restore this balance. 

Glutamate is a chemical that has links to major depressive disorder (MDD). Ketamine is an antagonist of the NMDA receptor, a type of receptor for glutamate. The drug actually blocks these receptors, which then produces an increase in levels of glutamate. The burst of glutamate is brief, correlating with how long the dissociative effects of ketamine last.

According to the glutamate hypothesis of depression, clinical depression results from a malfunction in the brain mechanisms that regulate glutamate levels. The way that ketamine changes the brain seems to support this hypothesis. As the authors of a study published in Biological Psychiatry state, “The increase in glutamate release produced by ketamine seems to be essential for its antidepressant effects”.

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Ketamine and Synapses

Synapses are the small pockets of space between two cells (and also can mean between nerve cell and a gland or muscle cell.) This allows cells to pass electrical or chemical signals to each other. This is how cells communicate messages. Synapses also allow neurons to form circuits within the brain. Neuroscientist Ronald S. Duman has said the following:

“We and others in the field reasoned that ketamine, via this glutamate burst, could increase synaptic connections in brain regions known to undergo atrophy and loss of synapses in animals exposed to chronic stress and in depressed patients.”

In animal studies, Duman and other Yale researchers tested the effects of ketamine on synapses in the prefrontal cortex, an area of the brain that shrinks in patients with depression. “The results were astounding: a single dose of ketamine rapidly increased the number and function of these synapses,” says Duman. Also, the increase in the number of synapses occurred as early as two hours after a single dose of ketamine. This is consistent with the rapid antidepressant effects of the drug.

A single dose of ketamine can reverse the shortage of synapses from chronic stress. This helps behavioral issues such as anhedonia (the inability to experience pleasure, a common symptom of depression).

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How Ketamine Changes Brain Regions

Ketamine leads to changes in various brain regions. Illuminating results from studies show the following:

• Ketamine enhances neural responses to positive emotion in the right caudate in patients with depression. After ketamine, there is greater connectivity to positive emotions.
• Major depression decreases connectivity between the pre-frontal cortex/subcortex and the rest of the brain. However, this connectivity normalized after ketamine.
• Decreased amygdala activation predicted the antidepressant effects of ketamine. The amygdala is a brain region from fear responses, and its activity often has ties linked to depression. However, the amygdala is more famously linked with anxiety disorders such as PTSD.
• Decreased suicidality post-ketamine is correlated with decreased metabolism of glucose in the infralimbic cortex.
• Improvements in depression are significantly correlated with increased glucose metabolism in the superior temporal gyrus, middle temporal gyrus, and cerebellum, and with decreased metabolism in the parahippocampal gyrus and inferior parietal cortex.
• Decreased anhedonia following ketamine is related to increased glucose metabolism in the dorsal anterior cingulate cortex and putamen. The biggest improvements in depressive symptoms after ketamine are correlated with the largest metabolic increases in the right ventral striatum.

Furthermore, ketamine enhances structural plasticity in certain brain regions. Structural plasticity refers to the brain’s ability to change its physical structure. This is important in the context of depression. One study concludes the following.

“These results suggest that the prolonged antidepressant effects observed after a single infusion of ketamine in TRD [treatment-resistant depression] patients can be related to a transient enhancement of structural plasticity induced by a glutamate “burst” occurring not only in frontal and hippocampal neurons but also in mesencephalic DA neurons…Since MDD/TRD has been associated to defective neural plasticity, the structural plasticity induced by these treatments could be interpreted as a potential remediation of an underlying neurobiological mechanism that sustains depressive symptoms.”

Ketamine Switches Off The Brain

Researchers from the University of Cambridge have discovered that high doses of ketamine can temporarily switch off the brain. The researchers measured the brain waves of sheep sedated by the drug. Changes to these brain waves may explain the out-of-body experiences and state of complete oblivion (known as the ‘k-hole’) that ketamine can cause. Professor Jenny Morton, from the University of Cambridge’s Department of Physiology, Development and Neuroscience, said that “after the high dose of ketamine the brains of these sheep completely stopped. We’ve never seen that before. A few minutes later, their brains were functioning normally again — as if they had just been switched off and on.”

The researchers believe that this pause in brain activity may correspond to the experience of the k-hole, which is comparable to a near-death experience and associated with a feeling of peace and serenity. The purpose of this research was not to better understand the effects of ketamine, but to use the drug to “probe the brain activity in sheep with and without the Huntington’s disease gene,” said Morton. She also added the following.

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“But our surprising findings could help explain how ketamine works. If it disrupts the networks between different regions of the brain, this could make it a useful tool to study how brain networks function – both in the healthy brain and in neurological diseases like Huntington’s disease and schizophrenia.”

There’s Still Plenty of Positive Information To Learn

Indeed, studies have shown that ketamine can induce psychosis-like symptoms in both healthy volunteers and patients with schizophrenia. Other researchers share Morton’s belief that ketamine could help us better understand schizophrenia. Knowing how ketamine affects the brain is essential with respect to this objective.

Ketamine is a drug that affects the brain on many different levels. But we still don’t fully understand all the brain changes that occur after its administration, including all the alterations responsible for the compound’s psychedelic and therapeutic effects. Research continues to offer us surprising results on how exactly this drug works.

What New Research Has Revealed About How Ketamine Changes the Brain

The mechanistic picture of ketamine’s brain effects has sharpened considerably since this article was first written. The core mechanism described here — NMDA receptor antagonism triggering a glutamate burst, which stimulates BDNF release, which drives synaptogenesis in the prefrontal cortex and hippocampus — has been substantially confirmed and refined by optogenetic studies in animal models that isolated each step in the cascade. Researchers can now block specific steps in the sequence and observe that the antidepressant effect disappears, which strongly supports the causal chain rather than mere correlation. The glutamate hypothesis of ketamine’s action is now the dominant mechanistic framework in psychopharmacology, though debate continues about the precise contribution of each downstream step.

One of the most important findings of the past two years involves ketamine’s effects on the default mode network (DMN) — the set of brain regions active during self-referential thought, rumination, and mind-wandering. Multiple fMRI studies published in 2024 demonstrated that ketamine substantially reduces the hyperconnectivity of the DMN that is characteristic of depression. What makes this especially significant is the timing: the DMN changes persist well beyond the drug’s half-life (roughly 2–3 hours), which helps explain why ketamine’s antidepressant effects last days to weeks after a single infusion. The structural synaptogenesis catalyzed by the glutamate-BDNF cascade takes days to consolidate, and it appears that the DMN reorganization tracks with that timeline. This is direct neuroimaging evidence that ketamine is doing something structurally different from what an antidepressant that requires weeks of daily dosing does.

A discovery that has generated significant pharmaceutical interest is the role of an active metabolite called (2R,6R)-hydroxynorketamine (HNK). Early research suggested HNK might contribute to ketamine’s antidepressant effects while bypassing the NMDA receptor mechanism that produces dissociation. If true, that would mean it may be possible to develop a ketamine-like antidepressant that works in hours rather than weeks, without the dissociative effects that make ketamine a Schedule III controlled substance requiring clinic administration. Several research groups and pharmaceutical companies are developing HNK analogs and related compounds. As of mid-2026, HNK-derived candidates are in early-stage human trials; no approval is near, but the mechanistic rationale is considered strong enough to have attracted significant investment.

Understanding why protocols use six infusions over two to three weeks — rather than a single dose — comes directly from the synaptogenesis timeline. A single ketamine infusion triggers the BDNF-driven synaptogenesis cascade, but synaptic structural changes take days to weeks to consolidate into stable new connections. Repeated infusions during this consolidation window appear to reinforce the structural changes rather than simply repeating the initial signal. This is why the standard induction protocol (six infusions over two to three weeks) is not arbitrary convention — it reflects the biology of how long synaptogenesis takes to stabilize. Esketamine (Spravato, the FDA-approved nasal spray form of S-ketamine) follows a similar schedule for the same reasons; it is slightly more potent at NMDA receptors than racemic ketamine, but both produce comparable downstream BDNF and synaptogenesis effects. The choice between IV ketamine and Spravato comes down to clinical setting, insurance coverage, and patient-specific factors, not a meaningful difference in how they affect the brain.

Frequently Asked Questions

How does ketamine work differently from antidepressants like SSRIs?

SSRIs (selective serotonin reuptake inhibitors) work by increasing the availability of serotonin in the synapse, which gradually modulates mood-regulating circuits over four to eight weeks of daily dosing. Ketamine works through an entirely different pathway: it blocks NMDA receptors, which are glutamate receptors, triggering a rapid cascade that produces new synaptic connections in the prefrontal cortex within hours. Glutamate and GABA — the neurotransmitters targeted by ketamine’s mechanism — account for roughly 80% of brain synaptic activity; serotonin, by comparison, is a modulatory system with far fewer synapses. This difference in scale partly explains why ketamine’s effects are faster and more pronounced, though it also means the side effect profile is different — ketamine causes transient dissociation during infusion, which SSRIs do not. The two approaches are not competing; many patients use ketamine to achieve rapid stabilization and then continue maintenance with an SSRI or other medication.

Why does ketamine work so much faster than traditional antidepressants?

The speed difference comes from the mechanism. SSRIs require gradual changes in receptor sensitivity over weeks — they raise synaptic serotonin immediately, but the downstream effects on gene expression and receptor density that produce the antidepressant effect take weeks to develop. Ketamine, by contrast, triggers BDNF release within hours of administration, and BDNF drives rapid synaptogenesis — the physical growth of new synaptic connections — in the prefrontal cortex. fMRI studies show that the brain’s default mode network (which is hyperactive in depression, driving rumination and negative self-referential thought) begins to reorganize within a single session. Most patients with treatment-resistant depression who respond to ketamine report noticeable mood improvement within 24 hours of the first infusion, sometimes within hours. This speed is particularly important in patients with acute suicidal ideation, where weeks of waiting for an SSRI to work is not clinically acceptable.

What is BDNF and why does it matter for depression?

BDNF stands for brain-derived neurotrophic factor — often called a “fertilizer for neurons.” It is a protein that promotes the growth, survival, and differentiation of neurons and synaptic connections. Depression is associated with reduced BDNF levels and with atrophy (shrinkage) of specific brain regions, particularly the prefrontal cortex and hippocampus, which play central roles in mood regulation, memory, and stress response. Chronic stress lowers BDNF; antidepressants, exercise, and ketamine all increase it. Ketamine’s NMDA blockade triggers an unusually large and rapid BDNF release — the glutamate burst that follows NMDA inhibition acts on TrkB receptors (BDNF’s primary receptor), activating mTOR signaling, which then drives the construction of new dendritic spines within hours. This is the mechanistic explanation for why ketamine can reverse years of stress-related synaptic atrophy in a matter of sessions: it is directly stimulating the protein responsible for rebuilding that synaptic architecture.

Does ketamine permanently change the brain?

The synaptogenesis ketamine drives is a real structural change — new dendritic spines form and new synaptic connections are established. In animal models, these structural changes are visible on electron microscopy and persist for weeks after the drug clears. However, “permanent” is not quite the right framing: brain circuits are dynamic, and without maintenance factors (continued treatment, lifestyle changes, therapy), the new connections can be pruned back over time. Most patients find that the antidepressant effects of a ketamine induction course last weeks to months, not indefinitely. Maintenance infusions — typically monthly or as needed — are used to sustain and reinforce the synaptic changes. Importantly, this is not a sign of dependence; it reflects the same biology that underlies why any physical fitness requires ongoing exercise to maintain. The good news is that each maintenance infusion appears to rebuild and reinforce the same circuit — ketamine is not simply masking symptoms the way a daily pill might, but repeatedly rebuilding the structural substrate of mood regulation.

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