Racemic vs S-Ketamine vs R-Ketamine: Key Differences

TL;DR

Racemic ketamine, S-ketamine, and R-ketamine are three forms of the same molecule with different therapeutic profiles:

• Racemic ketamine — a 50/50 mix of the two enantiomers. Used off-label for depression via IV, oral, or sublingual dosing. Broadest evidence base, lowest cost, most widely used in clinical practice.
• S-ketamine (esketamine, branded as Spravato) — the only form FDA-approved for treatment-resistant depression. Roughly 3–4x the NMDA-receptor binding affinity of R-ketamine. Stronger dissociative effects. Must be administered in a certified clinic.
• R-ketamine — showed strong antidepressant effects in preclinical models and was hypothesized to be longer-acting with fewer side effects, but failed to outperform placebo in a 2022 phase 2a trial. Not available outside research.

For most patients in real-world ketamine-assisted therapy, the form actually delivered is racemic ketamine. Spravato is the answer when insurance coverage is the deciding factor.

What are R-ketamine and S-ketamine?

Ketamine is a chiral molecule — its atoms can be arranged into two mirror-image structures that are not superimposable, like a left and a right hand. These two forms are called enantiomers. The right-handed form is R-ketamine (arketamine); the left-handed form is S-ketamine (esketamine). They share the same molecular formula but interact differently with brain receptors.

Racemic ketamine is a 50/50 mixture of the two. It is the form ketamine has been manufactured as since the 1960s.

Why does the difference matter?

Each enantiomer binds the NMDA receptor with different strength and triggers different downstream effects:

• S-ketamine binds NMDA receptors with roughly 3–4x the affinity of R-ketamine. This makes it more potent per milligram and explains its faster onset and stronger dissociative effects.
• R-ketamine has lower NMDA affinity but engages other mechanisms — including BDNF signaling and AMPA receptor activation — that contribute to neuroplasticity. In animal models it produced longer-lasting antidepressant effects with milder side effects.

A short history of ketamine in psychiatry

Ketamine was first synthesized in 1962 by Calvin Stevens at Parke-Davis. The FDA approved it as a surgical anesthetic in 1970, and it has been on the WHO Essential Medicines List ever since.

Its antidepressant potential was demonstrated in two foundational human trials:

• Berman et al., 2000 — the first controlled trial showing rapid antidepressant effects of a single IV ketamine dose in major depression (Biological Psychiatry)
• Zarate et al., 2006 — the landmark study showing antidepressant response within 2 hours of infusion in treatment-resistant depression (Archives of General Psychiatry)

Those two studies established ketamine as the first rapid-acting antidepressant and seeded two decades of clinical and pharmaceutical work.

What is racemic ketamine?

Racemic ketamine is the standard pharmaceutical form — equal parts R- and S-enantiomers. It is delivered in three main ways:

• IV infusion — typical clinical protocol, 40-minute session at 0.5 mg/kg
• Intramuscular injection — less common, sometimes used in research
• Oral or sublingual — the format used in most at-home ketamine therapy programs

Racemic ketamine is used off-label for depression, anxiety, PTSD, and other treatment-resistant mood disorders. Because it lacks FDA approval for psychiatric indications, it is generally not covered by insurance, but it costs substantially less than Spravato.

What is S-ketamine (Spravato)?

S-ketamine is the left-handed enantiomer and the active ingredient in Spravato, a nasal spray that received FDA approval in March 2019 for treatment-resistant depression.

Key facts:

• Roughly 3–4x the NMDA receptor binding affinity of R-ketamine
• Administered intranasally at 56 mg or 84 mg per session
• Must be given at a certified treatment center under the Spravato REMS program
• Patient must remain on-site for at least 2 hours of monitoring per session
• Typical out-of-pocket cost: $600–900 per session; partial insurance coverage is common when prescribed for TRD

The supervised-administration requirement makes Spravato more expensive and less convenient than at-home racemic ketamine, but the FDA approval is what drives insurance coverage and makes it the deciding factor for many patients.

What is R-ketamine?

R-ketamine — also called arketamine — is the right-handed enantiomer. Preclinical research made it look like the better of the two enantiomers for psychiatry:

• Stronger and longer-lasting antidepressant effects in animal models
• Lower dissociation and less abuse-related signal
• Potentially stronger neuroplasticity effects in the prefrontal cortex and hippocampus

The animal data drove serious pharmaceutical interest. Perception Neuroscience (an atai Life Sciences company) developed an R-ketamine formulation called PCN-101 and advanced it into human trials.

The most rigorous human test of R-ketamine was negative. PCN-101 failed to separate from placebo on its primary endpoint in a phase 2a trial in treatment-resistant depression, announced in November 2022. The result has tempered the field's enthusiasm; R-ketamine is not currently available outside of research settings.

How do the three forms compare?

Feature	Racemic ketamine	S-ketamine (Spravato)	R-ketamine
Composition	50% R + 50% S	Pure S-enantiomer	Pure R-enantiomer
NMDA receptor binding	Mixed	High (3–4x R)	Lower
FDA approval	No (off-label use)	Yes — for TRD	No
Onset of antidepressant effect	Hours	Hours	Days (animal data)
Dissociative side effects	Moderate	Stronger	Milder (preclinical)
Administration	IV, IM, oral, sublingual	Intranasal, in clinic	Not available
Insurance coverage	Rare	Common with TRD diagnosis	N/A
Typical patient cost	$400–800/IV; ~$398/mo at-home oral	$600–900 per session	N/A
Status	Most widely used off-label form	Standard FDA-approved option	Failed phase 2a

Which form should you choose?

This is a clinical decision that depends on four practical factors:

• Diagnosis. A formal treatment-resistant depression diagnosis is what unlocks insurance coverage for Spravato. Racemic ketamine is the more flexible option for broader indications.
• Tolerability. Patients sensitive to dissociation may do better on a lower-potency racemic protocol than on Spravato. A lower starting dose can be titrated up.
• Convenience. Spravato requires 2-hour in-clinic monitoring per session. Oral or sublingual racemic ketamine can be self-administered at home with telehealth supervision.
• Cost. Out-of-pocket cost varies widely. Get an itemized estimate before committing to any protocol. Compare with the Isha Health pricing page.

The right answer is best decided with a qualified provider who can review your history.

Neuroplasticity: a key mechanism

Neuroplasticity — the brain's ability to form new connections and rewire existing ones — is the leading hypothesis for why ketamine works as a rapid antidepressant. The three forms engage this mechanism differently:

• Racemic ketamine activates both pathways (via the R and S components) and is what most of the human neuroplasticity evidence is based on.
• S-ketamine strongly stimulates BDNF release and AMPA receptor activation, which drive new synaptic connections within hours of administration.
• R-ketamine (in animal models) produced more sustained synaptogenesis in the prefrontal cortex and hippocampus — the regions most implicated in mood regulation and cognitive flexibility.

Whether R-ketamine's preclinical neuroplasticity advantage would have translated to humans remains an open question after the phase 2a failure.

FAQs

Is S-ketamine the same as Spravato?

Spravato is the brand name for FDA-approved esketamine nasal spray. Esketamine and S-ketamine are the same molecule. Spravato is a specific pharmaceutical formulation with standardized 56 mg / 84 mg dosing and REMS-mandated in-clinic supervision; generic S-ketamine in compounded preparations may differ in formulation, concentration, and route of administration.

Why is R-ketamine not commercially available?

R-ketamine looked promising in animal studies but failed a 2022 phase 2a trial in treatment-resistant depression (PCN-101, Perception Neuroscience). Without efficacy data in humans, no formulation has reached regulatory approval. Research continues, but R-ketamine is not currently available clinically.

Is ketamine therapy covered by insurance?

Spravato has the broadest insurance coverage because it is FDA-approved for treatment-resistant depression. Racemic ketamine used off-label for depression is generally not covered, though most providers accept HSA/FSA. R-ketamine is not approved and not covered.

Which form is most effective?

Racemic ketamine has the broadest evidence base for off-label depression treatment and is the most widely used. S-ketamine (Spravato) has the highest-quality FDA-grade evidence but is indicated specifically for TRD. R-ketamine has no human efficacy data.

What are the side effects of ketamine?

Common short-term effects include dissociation, dizziness, nausea, transient blood pressure elevation, and impaired motor coordination. These typically resolve within a few hours. S-ketamine produces stronger dissociation than racemic ketamine; R-ketamine produced milder dissociation in animal studies. Long-term safety with appropriate medical supervision is favorable; high-frequency recreational misuse has been associated with bladder dysfunction.

What is the difference between ketamine and esketamine?

Ketamine without a prefix refers to racemic ketamine — the 50/50 mixture of both enantiomers. Esketamine is the S-enantiomer alone, sold as Spravato. Esketamine binds NMDA receptors more strongly per milligram; racemic ketamine has a longer clinical track record and is significantly less expensive.

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Related Articles

• Unlocking the Brain: How Psychedelics Drive Neuroplasticity for Mental Health
• How Ketamine Helps the Brain Heal: A Look at Its Long-Term Molecular Effects
• How does ketamine work?

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