Esketamine is not weaker than ketamine in a straightforward way. At the receptor level, esketamine actually binds about four times more strongly to its main brain target than ketamine’s other half does. But when it comes to treating depression, the picture flips: racemic ketamine (the standard form containing both mirror-image halves of the molecule) appears to work just as well or possibly faster, largely because of how the two drugs are delivered and what happens after they enter the body.
The answer depends entirely on what you mean by “weaker”: receptor binding, anesthetic power, antidepressant effect, or speed of relief. Here’s how the two compare on each front.
How the Two Forms Differ at the Molecular Level
Ketamine exists as two mirror-image molecules, called S-ketamine (esketamine) and R-ketamine. Standard ketamine is a 50/50 mix of both. Esketamine, sold as the nasal spray Spravato, is pure S-ketamine.
Esketamine binds roughly four times more tightly to NMDA receptors, the brain’s main glutamate-blocking targets thought to drive rapid antidepressant effects. It’s also three to four times more potent as an anesthetic. By these raw pharmacological measures, esketamine is the stronger molecule, not the weaker one.
Yet stronger receptor binding doesn’t automatically translate into a better antidepressant. In animal models of depression, the “weaker” R-ketamine half actually produced more potent and longer-lasting antidepressant effects than esketamine at the same dose. R-ketamine also did a better job restoring lost connections between brain cells in the prefrontal cortex and hippocampus of stressed mice, and it caused fewer psychosis-like side effects and showed lower abuse potential. Racemic ketamine delivers both halves at once, which may explain why it holds its own against the supposedly stronger isomer.
Delivery Method Changes Everything
One of the biggest practical differences has nothing to do with the molecules themselves. Esketamine is given as a nasal spray, and only about 45% to 50% of the dose actually reaches your bloodstream. Intravenous ketamine, by definition, delivers 100% of the dose directly into circulation. That gap in bioavailability means a significant portion of each esketamine spray never makes it to the brain.
This is a key reason people perceive esketamine as “weaker.” Even though the S-isomer binds more tightly to receptors, you’re absorbing roughly half of what you’re given through the nose. IV ketamine sidesteps that problem entirely.
Depression Outcomes: Similar Results, Different Speed
A systematic review and meta-analysis pooling data from multiple studies found no statistically significant difference in response or remission rates between IV ketamine and intranasal esketamine for depression. The odds of responding were slightly higher with IV ketamine, but not enough to reach statistical significance.
Where IV ketamine does pull ahead is speed. In one detailed comparison, the median number of treatments needed to achieve a clinical response was 2 for IV ketamine versus 4 for intranasal esketamine. For full remission, the gap widened further: a median of 2 IV ketamine sessions compared to 7 esketamine sessions. Time to remission was significantly shorter with IV ketamine, roughly five times faster based on hazard ratio analysis.
So while the two treatments eventually reach similar endpoints, IV ketamine gets patients there with fewer sessions. Again, much of this likely reflects the delivery route rather than the molecule itself.
Dosing and What a Session Looks Like
A typical IV ketamine infusion for depression uses about 0.5 mg/kg of body weight, infused over 40 minutes. Research suggests doses as low as 0.2 mg/kg can produce antidepressant effects, though 0.5 mg/kg appears to be the sweet spot. Going above that doesn’t seem to add benefit.
Esketamine nasal spray is dosed at 56 or 84 mg per session. During the first month, patients typically receive two sessions per week, then taper to weekly or every-other-week dosing. The effective range is 56 to 84 mg; a lower 28 mg dose doesn’t work as well. Each session requires at least two hours of monitoring in a certified healthcare setting for sedation, dissociation, and blood pressure changes.
Side Effects Are Comparable
Both forms cause dissociation, the floaty, out-of-body feeling that’s one of ketamine’s hallmark effects. Studies have found that dissociative symptoms from 84 mg of intranasal esketamine are comparable in severity to those from a standard IV racemic ketamine infusion. Blood pressure spikes, nausea, and headaches occur with both, though inconsistent reporting across studies makes precise head-to-head comparisons difficult.
Animal research does suggest that the S-isomer (esketamine) carries more psychosis-like side effects and higher abuse potential than the R-isomer. Since racemic ketamine contains both isomers, it delivers a lower proportion of the S-form per dose, which could theoretically mean a somewhat different side-effect profile in practice.
Long-Term Maintenance With Esketamine
Esketamine has the most robust long-term data because it went through the full FDA approval process. In the SUSTAIN-3 study, about 36% of patients with treatment-resistant depression were in remission after the initial induction phase. By the one-year mark, that number climbed to roughly 51%, suggesting continued improvement over time for those who stick with treatment. Dropout rates remained relatively steady, with about 9% of participants discontinuing in each six-month window.
Cost Is a Major Practical Difference
The price gap between the two is enormous. Based on Canadian formulary data (which closely mirrors U.S. pricing patterns), the drug cost alone for esketamine runs roughly $1,100 to $1,365 per week during the initial phase. The generic ketamine used in IV infusions costs between $5 and $16 per week in raw drug costs. Of course, IV infusions carry their own clinic fees for administration and monitoring, but even with those added, a ketamine infusion session typically costs a fraction of what a Spravato session does.
Insurance coverage further complicates the picture. Spravato, as an FDA-approved treatment, is more likely to be covered by insurance plans, while off-label IV ketamine infusions are often paid out of pocket. The out-of-door cost a patient actually faces can vary dramatically depending on their coverage.
So Which One Is Actually “Weaker”?
Esketamine is pharmacologically stronger than racemic ketamine at the receptor level and as an anesthetic. But for depression treatment, the intranasal delivery cuts its effective dose roughly in half before it even reaches the brain, and IV ketamine achieves response and remission in fewer sessions. The two land at similar overall outcomes, just on different timelines. Calling esketamine “weaker” oversimplifies a comparison that hinges more on how the drug gets into your body than on the molecule itself.