- Understanding Ketamine & its Mirror-Image Forms (S & R)
- What Does the Research Say About S-Ketamine for Depression?
- Research Limitations & Questions
- What This Means for Patients Considering Ketamine Therapy
Ketamine exists in two forms—S-ketamine and R-ketamine—and each may affect depression in different ways. S-ketamine is the faster-acting option and is FDA-approved as Spravato for treatment-resistant depression. R-ketamine is in early research stages, but theories suggest it may provide longer relief with fewer side effects. The two forms can also feel different during treatment, with S-ketamine more likely to create a noticeable dissociative experience.
Understanding the differences between s and r ketamine helps you make sense of what interventions are available for depression today and what may shape the future of mental health care.
Understanding Ketamine & its Mirror-Image Forms (S & R)
Ketamine contains mirror-image molecules (enantiomers) called S-ketamine (esketamine) and R-ketamine (arketamine). (source) When you receive ketamine in a hospital for anesthesia or in a clinic for depression, you are typically receiving a 50/50 mixture of both, known as a racemic blend.
Ketamine r and s share the same chemical formula. However, their unique shapes determine how each one fits into and interacts with receptors in your brain.
Once in the brain, both s- and r-ketamine influence systems involved in mood, perception, and neural communication. Together, they drive the antidepressant and sensory effects that ketamine is known for. At the same time, each form appears to contribute something slightly different to the overall experience and response, which is why researchers continue to study them separately.
How S-Ketamine and R-Ketamine May Affect the Depressed Brain Differently
S-ketamine and R-ketamine differ primarily in how “tightly” they bind to NMDA brain receptors and potentially how long their antidepressant effects last. Both molecules trigger the release of glutamate, a chemical messenger that helps the brain regrow neural connections. However, they take different biological pathways to do so. (repeated source)
Esketamine acts as a high-potency “blocker” of NMDA receptors, leading to a rapid shift in glutamate across brain regions. Conversely, R-ketamine is a weaker NMDA binder but could be a more potent stimulator of the long-term growth factors that keep the brain resilient against depression.
S-ketamine mechanisms
S-ketamine binds to NMDA receptors nearly four times more strongly than the R-form. (repeated source) This high affinity allows it to trigger an immediate antidepressant signal and dissociative “out-of-body” sensations.
Esketamine is highly effective. However, some theorize that s-ketamine’s effects may not last as long as r-ketamine’s because it creates a temporary shift in brain signaling rather than supporting longer-term changes in how brain cells connect and function.
R-ketamine mechanisms
Arketamine has a much weaker grip on NMDA receptors, leading to almost no dissociative high and a more grounded patient experience.
Pre-clinical research suggests r-ketamine could also be superior at activating BDNF (brain-derived neurotrophic factor), a protein that acts like “fertilizer” for neurons. This suggests that R-ketamine may be better at promoting the long-term structural brain changes needed to keep depression in remission. (source)
S-ketamine vs R-ketamine effects
The “high” from s-ketamine vs r-ketamine effects differs in intensity, clarity, and how detached or grounded patients feel during treatment.
Esketamine Effects
- creates a noticeable sense of dissociation, such as feeling separated from the body or surroundings
- can shift perception of time, space, and sensory input in a more noticeable way
- symptoms typically appear within minutes of the first dose and resolve naturally within 90 to 120 minutes.
- most common adverse events include dissociation, nausea, vertigo, dysgeusia, and dizziness
Arketamine Effects
- appears to produce a milder, more relaxed mental state in early research
- may involve subtle shifts in mood and perspective without strong detachment
- tends to feel less disruptive to awareness while still supporting antidepressant effects
- common side effects are primarily limited to mild blurred vision or dizziness without intense mental disruption.
What Does the Research Say About S-Ketamine for Depression?
Esketamine became a prescription medication (Spravato) for depression in 2019 after large-scale clinical trials confirmed its safety and efficacy.
Clinical Trials Supporting Depression Relief
Phase 3 efficacy and safety trials, such as the TRANSFORM programs, showed that patients who added esketamine nasal spray to a new oral antidepressant saw a significantly greater reduction in symptoms than those using a placebo. Those who continued with treatment for an additional four weeks saw even greater improvement.
The relapse prevention Phase 3 study (SUSTAIN-1 trial) showed that sketamine patients who achieved stable remission were 51% less likely to relapse than those who switched to a placebo.
Long-term data from Phase 3 maintenance studies (SUSTAIN-2 and SUSTAIN 3) confirmed that s-ketamine’s antidepressant effects can endure for at least one year after treatment without any lasting cognitive impairment.
Newer 2026 real-world findings (source) now extend these observations to 2.5 years of esketamine treatment. 85% of patients in the study showed improvement in depression, and 65% saw a significant drop in anxiety.
Research Limitations & Questions
Not every esketamine trial showed the same level of benefit across all patient groups. Additionally, questions remain about how it compares to other forms of ketamine.
For instance, in the TRANSFORM-3 trial in adults aged 65 and older with TRD, esketamine plus a new oral antidepressant did not show a clear advantage over placebo. Some patients still improved, but the results suggest that age and overall health can influence how well treatment works.
Additionally, a 2025 study from Mass General Brigham found that IV ketamine actually led to a 49.2% reduction in depression scores, whereas esketamine saw a 39.6% reduction. Patients in that study also reported faster relief with IV ketamine, sometimes after a single session.
R-Ketamine Depression Research
Arketamine research has grown as scientists look for ways to treat depression with fewer side effects.
Preclinical and Early Clinical Findings
In animal models of depression, arketamine has consistently shown stronger and longer-lasting antidepressant-like effects compared to S-ketamine. These studies also suggest fewer dissociative and behavioral side effects, which could make treatment feel more manageable. (source)
Early human data is more limited but still informative. A small pilot study in patients with treatment-resistant depression found that a single dose of intravenous arketamine led to swift improvements in depression symptoms after one day. The study reported minimal dissociation during treatment. (source)
Additional case reports have followed TRD patients after arketamine treatment and observed sustained improvements in mood and social and vocational functioning. All three patients went from needing significant time off work (up to 5 months in one case) to zero sick leave in the year following the single infusion. (source)
Research Limitations & Questions
Despite strong early signals, larger r-ketamine clinical trials have not yet confirmed an antidepressant advantage.
In a Phase 2a trial (PCN-101) involving over 100 patients with TRD, arketamine did not show a clear improvement over placebo at the 24-hour timepoint. Some improvement appeared later. However, the results were not strong enough.
Most human studies on arketamine are small, early-stage, or exploratory, which makes it difficult to draw firm conclusions about how well it works for depression. There is also still limited data on optimal dosing, treatment frequency, and long-term outcomes.
Table: Differences Between S and R Ketamine at a Glance
| Feature | S-Ketamine (Esketamine) | R-Ketamine (Arketamine) |
| Current Status | FDA-approved (as Spravato nasal spray). | Emerging option (currently in clinical trials). |
| NMDA Receptor Grip | High Potency: Binds nearly 4x more strongly to receptors. | Low Potency: Has a much weaker grip on receptors. |
| Primary Brain Goal | Focuses on a rapid chemical “blockade” to quickly lift mood. | Focuses on long-term repair by stimulating growth factors, such as BDNF. |
| The “High” Experience | Moderate to High: Noticeable dissociation, “out-of-body” feelings, and sensory shifts; resolves in ~2 hours. | Grounded: Milder mental state with fewer dissociative or “trippy” side effects. |
| Speed of Relief | Often peaks within 2-24 hours of the first dose | Early trials did not show a major difference at the 24-hour mark. May act as a slow builder over 1–2 weeks. |
| Duration of Relief | Highly effective for immediate relief, with many patients remaining in remission over 16 weeks with continued dosing (SUSTAIN 1) | Animal studies suggest it could last longer, but human trials haven’t confirmed this yet. |
| Clinical Evidence | Supported by large-scale Phase 3 trials (TRANSFORM and SUSTAIN). | Supported by strong preclinical data and small human pilot studies. |
| Known Limitations | May be less effective in older adults (65+); higher risk of nausea and vertigo. | Larger trials (like PCN-101) have not yet confirmed a clear advantage over placebo at 24 hours. |
(Source A, Source B, Repeated Source)
Why Pharmaceutical Companies Focused on Esketamine First
Pharmaceutical companies focused on esketamine before racemic ketamine and r-ketamine because it offered a more practical path to drug approval. As a single enantiomer, s-ketamine could be packaged into a patented nasal spray with standardized dosing and tested in large, regulated trials. Its stronger NMDA receptor activity also made it an appealing candidate. (source)
Key reasons s came before r
- s-ketamine could be developed as a proprietary single-molecule product rather than a generic racemic drug
- it fit a standardized nasal spray model that worked well for large clinical trials
- it already showed enough antidepressant activity to justify major investment
- arketamine remained mostly a preclinical candidate while esketamine advanced through phase 3 and phase 4 studies
What This Means for Patients Considering Ketamine Therapy
If you are considering ketamine therapy for depression, the key difference between S-ketamine vs R-ketamine effects comes down to what is available now. Today, treatment involves FDA-approved esketamine (Spravato) or racemic IV infusions. Arketamine is not yet available outside of research settings.
How to think about R vs S ketamine for depression in real-world care.
- S-ketamine (esketamine) offers the most established, regulated treatment option with insurance coverage for qualifying conditions.
- Racemic ketamine is commonly used in mental health clinics and includes both ketamine r and s working together.
- R-ketamine is not yet available, but ongoing research is exploring its potential.
If you are considering esketamine nasal spray or IV ketamine therapy, connect with an Avesta Ketamine and Wellness provider who can guide you through your options and help you choose the right path forward.
FAQs
What is the difference between esketamine and arketamine?
Esketamine and arketamine are two parts of the same ketamine molecule that affect the brain in different ways. Esketamine is FDA-approved and has strong clinical evidence for depression, while arketamine is still being studied and may offer longer-lasting effects with fewer side effects.
Is ketamine stronger than arketamine and esketamine?
“Stronger” depends on what you are measuring. Esketamine is the most potent chemically. It binds to the brain’s NMDA receptors about 3 to 4 times more tightly than the other forms. Arketamine is the weakest, but emerging research suggests it could be the strongest for promoting long-term brain healing and neuroplasticity with the fewest side effects. Large-scale studies must prove this theory.
Why is esketamine FDA-approved, but ketamine infusions are not?
No. Arketamine is still in the research phase and is only available in clinical trials. It has not been approved for depression treatment.
