How Ketamine for Depression Works and How Long It Lasts

Ketamine, originally developed as a surgical anesthetic in the 1960s, has emerged as one of the fastest-acting treatments for depression, with effects often detectable within hours rather than the weeks required by conventional antidepressants. A meta-analysis of trials in treatment-resistant depression found a strong effect within four hours of a single intravenous infusion, peaking at around 24 hours.¹ The science behind why it works, who it works best for, and how to sustain its benefits is still catching up with the clinical demand, and the practical landscape of ketamine treatment involves trade-offs that are worth understanding before walking into a clinic.

How Ketamine Affects the Brain

Ketamine’s primary known action is blocking a receptor called the NMDA receptor, which plays a central role in how brain cells communicate using the chemical messenger glutamate. When ketamine blocks these receptors, it triggers a cascade of downstream events, including a burst of glutamate release in the prefrontal cortex. One study in humans using specialized brain imaging confirmed a roughly 13% increase in a marker of glutamate cycling in the prefrontal cortex after ketamine administration compared to placebo.² That glutamate surge is thought to activate another type of receptor (AMPA receptors), which then promotes the growth of new connections between neurons.

The connection-building part appears to be critical. In rodent studies, ketamine rapidly increases the release or production of brain-derived neurotrophic factor (BDNF) and vascular endothelial growth factor (VEGF) in the prefrontal cortex and hippocampus, leading to growth of new synaptic spines and enhanced neurogenesis.³ Depression is associated with the loss of these connections in key brain regions, so ketamine may be doing something like rebuilding circuitry that has atrophied.

But the picture is not as neat as “block NMDA receptors, get better.” Research has shown that some of ketamine’s effects on nerve cell activity persist even when NMDA receptors are separately blocked by a different drug, which means something beyond simple NMDA blockade is at work.⁴ More provocatively, a small crossover trial found that when patients received the opioid-blocking drug naltrexone before their ketamine infusion, the antidepressant effect was dramatically blunted.⁵ Follow-up work in rats confirmed that the opioid system is necessary for ketamine’s antidepressant action, though ketamine does not behave like a traditional opioid and does not produce the same kind of euphoria or reward.⁶ The opioid system seems to be permissive rather than the main driver, but the finding is a reminder that ketamine’s mechanism is more complex than early accounts suggested.

Speed and Strength of the Antidepressant Effect

The defining feature of ketamine for depression is how quickly it works. With standard antidepressants, you wait weeks hoping the medication will take hold. With ketamine, changes in mood can begin within an hour of an intravenous infusion. In one clinical study, patients showed significant improvement in depression and anxiety scores at the one-hour mark after the first dose.⁷ The effect typically peaks around 24 hours and remains present, though diminished, about a week later.⁸ Multiple infusions over a course of treatment tend to deepen and prolong the benefit beyond what a single dose achieves.

A large randomized trial published in the New England Journal of Medicine compared ketamine infusions directly with electroconvulsive therapy (ECT) in patients with treatment-resistant major depression. About 55% of patients in the ketamine group responded to treatment, compared with 41% in the ECT group, and ketamine was statistically non-inferior to ECT.⁹ That result surprised many clinicians, because ECT has long been considered the gold standard for the most severe, drug-resistant depression. A separate meta-analysis, however, found that when pooling all available comparative trials, ECT still showed a moderate advantage over ketamine for overall depression severity, though ketamine had a milder side-effect profile in some respects, particularly regarding headache and muscle pain.¹⁰ Neither group showed significant differences in cognitive effects or serious adverse events. The bottom line for patients weighing these options: ketamine is a credible alternative to ECT, with a different side-effect trade-off, though the evidence base is still maturing.

Rapid Reduction in Suicidal Thinking

One of ketamine’s most striking clinical applications is its effect on suicidal ideation. When someone walks into an emergency department in acute suicidal crisis, most available treatments take weeks to work. Ketamine can begin to reduce suicidal thoughts within 90 minutes. In a proof-of-concept emergency department trial, 88% of patients who received a ketamine infusion achieved remission of suicidal ideation at 90 minutes, compared with 33% given placebo.¹¹

A larger double-blind trial in patients hospitalized for severe suicidal ideation found that 63% of those receiving ketamine achieved full remission of suicidal thoughts by day three, versus about 32% on placebo. The separation between groups was visible as early as 40 minutes after infusion and persisted over the three-day observation period.¹² A meta-analysis looking across multiple trials reported that ketamine was significantly more effective than placebo, esketamine, and midazolam (a sedative sometimes used as an active control) at reducing suicidal ideation on day one, and maintained its advantage over placebo through at least the first week.¹³ For emergency psychiatry, this speed matters enormously.

IV Ketamine, Nasal Esketamine, and Oral Forms

Ketamine is a molecule with two mirror-image forms. The version used in most research and off-label treatment is “racemic” ketamine, a 50/50 mix of both forms, delivered intravenously (IV). In 2019, the FDA approved esketamine, the S-form only, as a nasal spray (brand name Spravato) specifically for treatment-resistant depression. These are different products with different regulatory paths, different costs, and possibly different effectiveness.

A systematic review comparing racemic IV ketamine with intranasal esketamine found that racemic ketamine had substantially higher response and remission rates, though the comparison is complicated because the two forms were studied in different trial populations and designs.¹⁴ A more recent meta-analysis pooling head-to-head data found that IV ketamine had a numerical advantage in both response and remission rates, but the differences were not statistically significant.¹⁵ A retrospective chart review comparing the two in real-world practice found that IV ketamine patients achieved about a 49% reduction in depression scores by the eighth session versus about 40% for intranasal esketamine patients, with the IV group reaching significant improvement one session earlier.¹⁶

Oral ketamine is the most practical option on the horizon, since it does not require IV lines or clinic visits for nasal spray administration. A meta-analysis covering nearly 600 patients across randomized controlled trials of oral ketamine found it to be effective, with a number needed to treat of about five for response.¹⁷ However, individual trials have struggled to hit their primary statistical endpoints, possibly because oral ketamine is heavily converted to the metabolite norketamine before reaching the brain, producing a different pharmacological profile. A pair of randomized trials testing a prolonged-release oral ketamine tablet at 240 mg per day found rapid improvements in depression scores within hours, reaching statistical separation from placebo by day four, though the primary endpoint at day 21 was not met.¹⁸ Oral forms are still being refined, and no oral ketamine product has FDA approval for depression yet.

Ketamine in Bipolar Depression

Bipolar depression is notoriously harder to treat than unipolar depression, partly because many antidepressants risk triggering a switch into mania. The evidence for ketamine in bipolar depression is smaller but encouraging. An updated systematic review found that about 48% of patients with bipolar depression receiving ketamine achieved a response, compared with 5% on placebo. Response rates were lower in real-world studies (around 30%) than in clinical trials (around 63%), a gap that likely reflects the messier conditions of everyday practice.¹⁹

The concern about manic switching has been modest so far. Across the available evidence, a small fraction of patients with bipolar disorder developed hypomanic or manic symptoms after ketamine infusions. One scoping review identified three out of 66 bipolar patients who developed hypomania, all of whom were simultaneously taking antidepressant medications.²⁰ Overall about 2% of patients in the pooled literature developed manic or hypomanic symptoms. The risk appears real but low, and most clinicians administer ketamine for bipolar depression alongside a mood stabilizer as a precaution. The effect of a single dose does not seem to last beyond about two weeks, though repeated dosing schedules may extend it.²¹

Side Effects and Safety

During a typical subanesthetic infusion (the standard dose is 0.5 mg/kg over 40 minutes), the most common experiences are dissociation, dizziness, nausea, and a transient increase in blood pressure. The dissociation is often described as feeling detached from your body or surroundings, and it resolves within an hour or two. One study actually found that a specific aspect of dissociation, depersonalization, was positively associated with the antidepressant response, raising the question of whether the altered state is part of the therapeutic mechanism or just a side effect that happens to travel with it.²²

Blood pressure increases during infusions are generally mild and transient. A report spanning 684 infusions found average peak increases of only a few mmHg in systolic and diastolic pressure, with all readings returning to baseline during post-infusion monitoring.²³ People who already have high blood pressure do reach higher peaks, so monitoring is standard practice.²⁴

Bladder and Urinary Risks

A well-documented consequence of heavy recreational ketamine use is ketamine-induced cystitis, an inflammatory condition of the bladder that causes urinary pain, increased frequency, and in severe cases, damage to the bladder lining and even kidney problems. Regular recreational use increases the risk of cystitis symptoms by three to four times, and the condition can be seen in over a quarter of regular recreational users.²⁵ Recreational doses tend to be much higher and more frequent than therapeutic ones, which is an important distinction.

Until recently, ketamine-induced cystitis had not been reported in patients receiving therapeutic doses for depression. A 2023 case report documented the first known instance, describing a patient who developed bladder symptoms during a course of therapeutic ketamine infusions.²⁶ The case is a red flag worth tracking, especially as maintenance protocols extend to longer treatment durations. Clinicians increasingly recommend periodic screening for urinary symptoms in patients on long-term ketamine treatment, even though the therapeutic doses are far lower than those associated with damage in recreational settings.

Addiction and Misuse Potential

Ketamine is a Schedule III controlled substance in the United States, which puts it in the same regulatory category as testosterone and some sleeping medications. There is legitimate concern about misuse, especially as at-home oral ketamine prescribing through telehealth platforms has expanded. A systematic review examining addiction risk in patients receiving ketamine for depression found that only four patients across the entire reviewed literature showed clear signs of tolerance to the antidepressant effect or dependence on the drug, with the majority showing no such signs.²⁷ A separate analysis argued that ketamine’s misuse risks appear roughly comparable to other commonly prescribed medications with abuse potential, such as stimulants for ADHD or benzodiazepines for anxiety, and should be managed with similar prescribing caution rather than treated as categorically different.²⁸

The risk profile shifts depending on the setting. In-clinic IV infusions, where the drug is administered by a provider and you leave without a supply, carry a very different risk than a mail-order prescription for lozenges or tablets you take at home on your own schedule. Most of the addiction concern centers on unsupervised, frequent self-dosing.

How Long the Effects Last and Maintenance Dosing

The central frustration with ketamine for depression is that its effects fade. A single infusion typically produces peak benefit at about 24 hours, with the antidepressant effect declining over the next two to three days and largely gone by one to two weeks.²⁹ This temporal pattern maps onto the idea that ketamine opens a “plasticity window” in the brain lasting roughly two to three days, during which new synaptic connections form and consolidate. Once that window closes, the biological substrate for the mood improvement begins to erode unless reinforced.

This has led to the development of maintenance protocols, where patients receive periodic infusions or nasal spray doses after an initial acute treatment course. A systematic review of maintenance strategies found that patients on regular, fixed-interval dosing schedules had relapse rates of about 27%, compared with roughly 46% for those on variable or as-needed schedules.³⁰ Clinical evidence also suggests that twice-weekly and thrice-weekly dosing during the acute phase produce similar four-week outcomes, and that weekly maintenance sessions reduce relapse risk. There is no universal consensus yet on the ideal long-term schedule, and individual patients vary substantially in how quickly the benefit fades.

Does Adding Psychotherapy Help?

The idea of ketamine-assisted psychotherapy (KAP) is intuitive: if ketamine opens a window of brain plasticity and emotional openness, perhaps pairing it with structured therapy during that window would lock in deeper, more lasting changes. The evidence so far, though, has been surprisingly mixed. A systematic review found that in the small number of studies comparing ketamine alone to ketamine plus psychotherapy, the majority did not find that adding psychotherapy produced superior outcomes.³¹ A more recent study in a help-seeking clinical sample found no significant differences in symptom trajectories over 30 days between patients who received ketamine alone and those who received ketamine with concurrent psychotherapy.³²

This does not prove that psychotherapy adds nothing. The studies are small, the psychotherapy approaches used have been heterogeneous, and timing has varied. It is possible that the right type of therapy at the right moment during the plasticity window would show a clear benefit. But the current research does not support the marketing claim, common in ketamine clinic advertising, that KAP is substantially more effective than the drug alone. For now, the evidence suggests that ketamine’s pharmacological effects are doing the heavy lifting.

Cost, Access, and the Clinic Landscape

The practical barriers to ketamine treatment are significant. Intranasal esketamine (Spravato) is the only FDA-approved ketamine-based product for depression, and it must be administered in a certified healthcare setting under observation, which adds cost and logistical difficulty. Insurance coverage for esketamine exists but is often described as time-intensive and uncertain to secure. Off-label IV ketamine is almost entirely uninsured, with most patients paying out of pocket.³³

Demographic data from ketamine clinics reflect these economics: most patients fall into higher income brackets, and some providers have responded by offering tiered pricing or payment plans. The proliferation of ketamine clinics has been compared to the earlier wave of lidocaine infusion clinics, with similar concerns about inconsistent oversight.³⁴ There is no mandatory credentialing or standardized protocol for off-label ketamine clinics, and quality varies. This does not make ketamine ineffective, but it means patients need to do their homework on the specific clinic they are considering, including whether the provider monitors vitals during infusions, screens for contraindications, and has a plan for ongoing care beyond the initial infusion series.

What Brain Imaging Reveals

Neuroimaging studies have given researchers a clearer picture of what ketamine changes in the depressed brain. One of the most consistent findings involves the default mode network (DMN), a set of brain regions that is active during rest and self-referential thought and is often abnormally connected in people with depression. Two days after a ketamine infusion, connectivity between the insula and the DMN in depressed patients normalized to levels seen in healthy controls. The change was reversed by 10 days and did not appear in placebo scans, which aligns with the known time course of ketamine’s clinical effects.³⁵

Pre-treatment brain connectivity patterns may also predict who responds best. A study found that patients with stronger connectivity between certain prefrontal and insular regions before treatment showed greater reductions in depression symptoms after a series of infusions.³⁶ This is early work, but it points toward the possibility of using brain scans to identify patients who are most likely to benefit, rather than treating everyone and hoping for the best.

The Search for Blood-Based Biomarkers

A practical limitation of imaging-based prediction is that brain scans are expensive and inaccessible to most patients. Researchers have been searching for simpler biomarkers, ideally something measurable in a blood draw, that could predict who will respond to ketamine. A systematic review and meta-analysis of blood-based biomarkers for ketamine and esketamine response came to a sobering conclusion: there were no consistent associations between any baseline blood markers and treatment response.³⁷ Researchers have investigated inflammatory markers, stress hormones, neurotrophic factors, and genetic variants, but none has yet emerged as a reliable clinical predictor.³⁸ For now, the best predictor of whether ketamine will work for a given person is to try it, which underscores the need for accessible, safe treatment settings.