Ketamine is one of the most effective treatments available for depression that hasn’t responded to standard medications. In clinical studies, around 80% of patients with treatment-resistant depression achieved a meaningful response after a series of infusions, and the hallmark feature of ketamine is speed: symptoms can improve within hours of a single dose, compared to the weeks or months required by traditional antidepressants.
How Quickly Ketamine Works
Traditional antidepressants like SSRIs and SNRIs typically require several weeks of daily use before they produce a noticeable therapeutic effect, and many patients with depression never respond to them at all. Ketamine operates on a completely different timeline. A single low-dose IV infusion, delivered over about 40 minutes, can reduce depression scores within four hours. That effect can last up to three days from just one session.
This rapid onset makes ketamine especially valuable in acute situations. A meta-analysis published in Translational Psychiatry found that ketamine significantly reduced suicidal thoughts within the first day after treatment. The improvement in suicidal ideation after a single day of ketamine was comparable to what untreated patients experienced after roughly 26 days of natural recovery. No other available antidepressant works this fast.
Response and Remission Rates
A naturalistic follow-up study tracking patients with treatment-resistant unipolar and bipolar depression found that after a series of repeated ketamine infusions, 80.3% of patients achieved a clinical response, meaning their symptoms dropped by at least half. The remission rate, representing patients whose depression essentially resolved, was 78.9%. These are strong numbers, particularly for a population that had already failed other treatments.
Among those who responded, about 46% sustained their improvement over a nine-month follow-up period while continuing some form of maintenance treatment. That means a little over half of initial responders did experience some degree of relapse over time, which is an important consideration when weighing long-term expectations.
How Long the Effects Last
A single ketamine dose produces effects that are powerful but short-lived. The average duration of relief from one infusion is about one week, and nearly all patients relapse within two weeks if they don’t receive additional doses. This is why ketamine is almost always given as a series rather than a one-time treatment.
A standard induction protocol involves six infusions spread over three weeks (twice per week). After this initial phase, patients who respond transition to a maintenance schedule with less frequent sessions. The spacing of maintenance infusions varies from person to person, and research suggests the frequency can be reduced over time without losing the benefit. In a randomized discontinuation study of esketamine (a closely related nasal spray), only about 25% of patients relapsed during the year they continued treatment. Those who stopped the esketamine but stayed on a standard oral antidepressant relapsed at more than double that rate, over 57%.
Why Ketamine Works Differently
Standard antidepressants increase levels of serotonin or norepinephrine in the brain and then wait for downstream changes to slowly take effect. Ketamine takes a fundamentally different path. It blocks a specific type of receptor (the NMDA receptor) on inhibitory brain cells, which causes a surge of glutamate, the brain’s primary excitatory chemical messenger. That glutamate surge triggers a chain reaction that ultimately promotes the growth of new connections between neurons, a process called synaptogenesis.
Think of it this way: depression is associated with a loss of neural connections in key brain regions involved in mood. Traditional antidepressants try to nudge the brain’s chemistry so it slowly rebuilds those connections over weeks. Ketamine essentially fast-tracks that rebuilding process, stimulating the brain’s growth and repair pathways directly. This is why relief can begin within hours rather than weeks.
IV Ketamine vs. Nasal Spray Esketamine
There are two main forms of ketamine used for depression. IV ketamine uses the full ketamine molecule administered through a vein in a clinic. Esketamine (sold as Spravato) is a nasal spray that contains only one half of the ketamine molecule and is the only form with FDA approval specifically for depression.
Spravato is approved for two uses: treatment-resistant depression in adults (either alone or alongside an oral antidepressant) and depressive symptoms in adults with major depressive disorder who have acute suicidal thoughts or behavior. The FDA label notes that its effectiveness in actually preventing suicide has not been demonstrated. IV ketamine, meanwhile, is used off-label for depression, meaning doctors can legally prescribe it but it hasn’t gone through the FDA approval process for this specific purpose.
In terms of how well they work, the two forms appear quite similar. A 2025 meta-analysis combining data from over 900 patients found no statistically significant difference in response or remission rates between IV ketamine and intranasal esketamine. IV ketamine showed a slight numerical edge, and some evidence suggests it may work a bit faster, but the differences were small and could easily be due to chance. For most patients, the choice between the two comes down to availability, insurance coverage, and clinical preference rather than a clear efficacy gap.
Side Effects During Treatment
Ketamine’s side effects are generally short-lived, peaking during the infusion or nasal spray session and fading within a couple of hours. The most common ones include dissociation (a feeling of being detached from your body or surroundings), dizziness, nausea, sedation, a temporary increase in blood pressure, headache, blurred vision, and in some cases anxiety or a feeling of intoxication. Because of the blood pressure changes and dissociation, patients are monitored in a clinical setting for at least two hours after each session and aren’t allowed to drive for the rest of the day.
These side effects are the main reason ketamine for depression is administered in a supervised clinical environment rather than at home. For most people, they’re uncomfortable but manageable, and they don’t intensify with repeated treatments.
Who It Works Best For
Ketamine’s most studied and most compelling use is in treatment-resistant depression, typically defined as depression that hasn’t improved after trying at least two different antidepressants at adequate doses. This is the population where 80% response rates have been documented, and it’s the group for which the FDA approved esketamine.
It’s also being used in urgent situations where someone is experiencing severe depressive episodes with suicidal thinking and cannot afford to wait weeks for a traditional antidepressant to take effect. The rapid reduction in suicidal ideation within 24 hours fills a gap that no other medication currently addresses. That said, ketamine is not a standalone crisis intervention. It works best as part of a broader treatment plan that includes ongoing therapy and, for most patients, a concurrent oral antidepressant to help sustain the gains between sessions.