Ketamine relieves depression through a fundamentally different mechanism than traditional antidepressants. Rather than slowly adjusting serotonin levels over weeks, ketamine triggers a rapid burst of new neural connections in brain regions weakened by chronic depression. Some patients notice improvement within hours, and most who respond do so within 24 to 72 hours, compared to the 4 to 8 weeks typical of standard antidepressants.
The Glutamate Surge
Traditional antidepressants work by keeping more serotonin available between nerve cells. Ketamine takes an entirely different route. It blocks a specific type of receptor (called NMDA) on inhibitory brain cells, the ones whose job is to quiet neural activity. When ketamine silences these “quieting” cells, the result is a sudden flood of glutamate, the brain’s primary excitatory chemical messenger, particularly in the prefrontal cortex.
That glutamate surge activates a second set of receptors (AMPA receptors) on nearby neurons, causing them to fire rapidly. This chain reaction is what kicks off the downstream effects that actually lift depression. Think of it like removing the brakes on a car that’s been stuck: the engine was always there, but something was holding it back.
Regrowing Lost Connections
Depression physically shrinks parts of the brain. Chronic stress and prolonged depressive episodes reduce the number of synaptic connections, especially in the prefrontal cortex and hippocampus. Neurons lose their dendritic spines, the tiny branches that allow one nerve cell to communicate with another. Over time, entire circuits thin out.
Ketamine reverses this. The glutamate surge triggers the release of brain-derived neurotrophic factor (BDNF) and other growth signals in the prefrontal cortex and hippocampus. These growth factors activate a cellular pathway called mTORC1, which acts like a construction foreman, directing the cell to build new proteins, assemble new synaptic connections, and restore dendritic spines. In rodent studies, new spine synapses begin forming within hours of a single dose. This process, called synaptogenesis, is likely what makes ketamine’s effects feel so rapid compared to medications that simply adjust neurotransmitter levels without directly rebuilding lost architecture.
Resetting Brain Network Activity
Beyond regrowing individual connections, ketamine appears to recalibrate how entire brain networks communicate. Depression is associated with overactivity in the default mode network, a collection of brain regions involved in self-referential thinking. In people with depression, this network tends to run on a loop, fueling rumination, self-criticism, and the feeling of being stuck inside your own head.
Imaging studies show that ketamine reduces connectivity within the default mode network while strengthening connections between the prefrontal cortex (responsible for planning and decision-making) and deeper emotional processing regions. One study found that increased connectivity between the lateral prefrontal cortex and a region called the subgenual cingulate, a key node in mood regulation, predicted who would respond best to treatment. In patients with major depression, ketamine also normalized the way the default mode network communicates with areas involved in bodily awareness, bringing activity patterns closer to those seen in healthy volunteers.
The net effect: less self-focused rumination, more capacity for cognitive control. Many patients describe this as a lifting of mental fog or a sudden ability to see problems from a new perspective.
How It’s Administered
Ketamine for depression is delivered in two main forms. The most studied approach is intravenous (IV) infusion, typically given at a low dose over 40 minutes. This is considered “off-label” use because the FDA has only approved ketamine as an anesthetic, not specifically for depression. The second form is esketamine (brand name Spravato), a nasal spray that uses one half of the ketamine molecule. Esketamine received FDA approval in 2019 for treatment-resistant depression and is administered in a certified healthcare setting.
During a typical IV course, infusions are given no more than twice per week over about four weeks. Each session lasts roughly 40 minutes, and you’ll be monitored afterward because of potential side effects. The experience during infusion can include mild dissociation, a floaty or detached feeling, along with some visual distortion. These sensations typically fade within an hour or two after the infusion ends.
How Long the Effects Last
This is the catch. While ketamine works fast, a single infusion generally wears off within about a week. The rapid-onset relief is real but temporary unless followed by additional treatment. Clinical trials have shown that a series of infusions over two to four weeks can produce a more sustained response, and that once-weekly maintenance infusions afterward are sufficient to preserve the antidepressant effect in many patients.
The goal of the initial series is to build up enough new synaptic connections and network changes that the brain has a more durable foundation. Maintenance infusions then keep those gains from eroding. How long someone needs maintenance treatment, and whether the brain eventually sustains improvements on its own, varies from person to person. Many clinicians combine ketamine with psychotherapy during the window of enhanced neuroplasticity, aiming to make cognitive and behavioral changes “stick” while the brain is most receptive.
Side Effects and Risks
The most common side effects during infusion are dissociation, nausea, headache, and anxiety. These are generally short-lived and resolve within a couple of hours. The more clinically significant concern is blood pressure. Ketamine stimulates the cardiovascular system, and 10 to 50 percent of patients experience a temporary rise in blood pressure during the infusion. Blood pressure typically peaks around the 40-minute mark, with average increases of about 16 points systolic and 11 points diastolic.
In a study of 84 patients across 205 infusions, roughly 20 to 30 percent experienced blood pressure readings above 180/100 or a heart rate above 110 beats per minute, and 12 of those patients needed medication to bring their blood pressure down. These spikes were most common during the first three infusions and in patients who were older or had a history of high blood pressure. Because of these cardiovascular effects, esketamine is contraindicated in people with cerebral aneurysms, a history of bleeding in the brain, or certain blood vessel malformations.
Long-term risks from repeated use at therapeutic doses are still being studied. At much higher doses used recreationally, ketamine is known to cause bladder damage and cognitive problems, but these effects have not been a major concern at the low doses used for depression. Still, this is part of why treatment is administered in medical settings with monitoring rather than prescribed for home use (with IV ketamine; esketamine also requires in-office administration).
Who It’s Typically Used For
Ketamine treatment is generally reserved for people with treatment-resistant depression, meaning they haven’t responded adequately to two or more standard antidepressants. It’s also used in cases of severe suicidal ideation, where the weeks-long wait for traditional medications to take effect poses a real danger. The rapid onset, sometimes within hours, makes it uniquely valuable in acute crises where time matters.
It is not a first-line treatment. The need for medical supervision, the temporary nature of each dose’s effects, the cardiovascular risks, and the limited long-term data all place it further down the treatment ladder. But for people who have cycled through multiple medications without relief, ketamine represents a mechanistically distinct option that works through neural growth and network repair rather than simply adjusting neurotransmitter levels.