How Is Ketamine Metabolized in the Body?

Ketamine is broken down primarily in the liver, where cytochrome P450 enzymes strip off a methyl group to produce norketamine, the drug’s first and most important metabolite. From there, the story branches into a surprisingly complex web of secondary and tertiary metabolites, some of which are pharmacologically active in their own right and are reshaping how researchers think about ketamine’s antidepressant effects. The metabolism of ketamine matters not just academically but practically, because genetic variation, sex, other medications, and duration of use can all shift how quickly and completely the drug is processed.

The First Step Happens in the Liver

Ketamine’s primary metabolic pathway is N-demethylation, a reaction that converts ketamine into norketamine. This reaction takes place in the liver and is carried out by a family of enzymes called cytochrome P450s. In human liver tissue, CYP3A4 is the principal enzyme responsible, with CYP2B6 and CYP2C9 playing smaller supporting roles at the drug concentrations typically seen during clinical use.¹ Both of ketamine’s mirror-image forms (the S and R enantiomers) follow this same hepatic route.²

Norketamine is not an inert waste product. It retains the ability to block NMDA receptors, the same molecular target that gives ketamine its anesthetic and dissociative effects, though it does so with lower potency. The S-form of norketamine binds NMDA receptors with roughly eight times greater affinity than the R-form.³ This means that even after the liver has begun dismantling the parent drug, pharmacologically relevant molecules are still circulating in the bloodstream.

What Happens After Norketamine

Norketamine is not the end of the line. It undergoes further chemical modifications that produce a family of downstream metabolites. Two of the most studied are hydroxynorketamine (HNK) and dehydronorketamine (DHNK). Norketamine gets hydroxylated at various positions on its cyclohexanone ring, producing several hydroxynorketamine forms. It can also be oxidized through dehydrogenation to produce dehydronorketamine.² Many of the hydroxylated products are then conjugated with glucuronic acid, which makes them water-soluble enough to be excreted in urine.

A pharmacokinetic modeling study described the full journey of ketamine through the body as requiring a seven-compartment model to account for ketamine itself, norketamine, hydroxynorketamine, and dehydronorketamine, with metabolic transition steps between them.⁴ In practical terms, this means that a single dose of ketamine generates a cascade of compounds that appear in the blood and brain at different times and persist for different durations.

Hydroxynorketamine and the Antidepressant Question

One of the more exciting developments in ketamine research over the past decade involves hydroxynorketamine, specifically the (2R,6R)-HNK form. A landmark 2016 study in mice found that metabolizing ketamine into this compound was essential for its antidepressant effects, and that (2R,6R)-HNK produced antidepressant-like behavior on its own. The mechanism appeared to work through activation of AMPA receptors rather than through blocking NMDA receptors, which is how ketamine itself operates.⁵ Crucially, (2R,6R)-HNK did not produce the dissociative or abuse-related side effects associated with ketamine.

More recent work has started to complicate and refine this picture. A 2024 mouse study found that the sustained antidepressant effect of (2R,6R)-HNK, as opposed to its immediate effect, was abolished in mice lacking a particular NMDA receptor subunit called GluN2D. The acute effect persisted, but the longer-lasting benefit disappeared.⁶ So even this “NMDA-independent” metabolite may depend on NMDA receptor subtypes for part of its action. The field has not reached a tidy conclusion yet, but the idea that ketamine’s metabolites, not just ketamine itself, drive its antidepressant benefits has become a serious line of research.

These Metabolites Reach the Brain

For metabolites to matter pharmacologically, they need to cross the blood-brain barrier. A study in healthy volunteers measured ketamine, norketamine, and hydroxynorketamine levels in both blood plasma and cerebrospinal fluid (the liquid surrounding the brain and spinal cord). The concentrations in cerebrospinal fluid closely mirrored those in plasma, with ratios of roughly 0.92 for ketamine, 0.83 for norketamine, and 0.98 for HNK.⁷ In other words, nearly all of these compounds cross into the central nervous system with high efficiency. This confirms that downstream metabolites are not just circulating in the blood waiting to be excreted; they are present in the brain at concentrations high enough to have biological effects.

Mirror Images Are Not Processed Equally

Ketamine exists as two mirror-image molecules: S(+)-ketamine and R(−)-ketamine. Clinically, S-ketamine is the form used in the nasal spray esketamine (Spravato), while most injectable ketamine is the racemic mixture containing equal parts of both. These two forms are not metabolized at the same rate. When given alone, S-ketamine is cleared from the body almost twice as fast as R-ketamine. But when both are present in the racemic mixture, R-ketamine slows down the clearance of S-ketamine, effectively competing for the same enzymatic machinery.⁸

This has practical implications. A patient receiving the racemic mixture will have higher circulating levels of S-ketamine than you would predict from its standalone clearance rate, because R-ketamine is gumming up the works. It also means the metabolite profile following a racemic dose differs from what you see after a pure S-ketamine dose. The downstream metabolites inherit this stereoselective twist: the ratio of S- to R-norketamine, and consequently of their hydroxylated products, shifts depending on what form of the parent drug was administered.

Your Genes Influence How Fast You Clear Ketamine

Because CYP2B6 contributes to ketamine’s breakdown, genetic variants of this enzyme can alter how quickly or slowly a person metabolizes the drug. A study in chronic pain patients found that individuals carrying two copies of the CYP2B6*6 variant had a median ketamine clearance of about 22 liters per hour, compared to roughly 68 liters per hour in those with the normal genotype, a threefold difference. Patients with one copy of the variant fell in between, at around 41 liters per hour.⁹ These slow metabolizers also had a higher ratio of ketamine to norketamine in their blood, meaning they were converting less of the drug to its first metabolite.

Laboratory work using purified enzyme variants tells a consistent story. Across a panel of CYP2B6 genetic variants, several showed less than half the metabolic activity of the normal enzyme, and two variants (CYP2B6.16 and CYP2B6.18) were essentially inactive. The order of metabolic speed was consistent regardless of which ketamine enantiomer was tested: S-ketamine was always broken down faster than R-ketamine, and the genetic variants did not alter this preference.¹⁰

However, the picture is not perfectly clean. A clinical study specifically designed to test whether CYP2B6 genotype affected ketamine metabolism found no significant difference between genotype groups in any of its outcome measures.¹¹ The discrepancy likely reflects the fact that CYP3A4, not CYP2B6, is the dominant enzyme at clinical concentrations. In other words, the CYP2B6 variant matters most when CYP3A4 is saturated or inhibited, as might happen at higher ketamine doses or with chronic use. Age compounds the effect: older patients tend to clear ketamine more slowly regardless of genotype.

Sex Differences in Metabolism

Men and women process ketamine differently. In a study comparing the two sexes, males had higher circulating levels of both ketamine and norketamine than females after the same dose, while hydroxynorketamine levels were similar between the groups.¹² Mouse experiments helped clarify the mechanism: castrating male mice produced a pharmacokinetic profile resembling that of females, and replacing testosterone reversed the change. Removing the ovaries in female mice, by contrast, did not alter their ketamine metabolism. This points to testosterone as a key driver of the sex difference, rather than estrogen.

The practical takeaway is that females convert ketamine to hydroxynorketamine more efficiently. If HNK is indeed critical to the antidepressant effect, this could partially explain clinical observations that some women respond differently to ketamine therapy than men, though the evidence connecting metabolism to clinical outcomes is still thin.

Metabolism Outside the Liver

While the liver is the main processing center, it is not the only organ that breaks down ketamine. A rat study tracking tissue uptake of ketamine enantiomers found that at high circulating concentrations, the kidney was actually the dominant site of uptake and metabolism, with smaller contributions from the lung and gut. After concentrations dropped during washout, the kidney and gut continued to dominate metabolic activity.¹³ The kidney’s role is particularly interesting given the well-documented bladder toxicity seen in heavy or chronic ketamine users.

Ketamine metabolites can cause severe inflammation of the bladder lining, damage to the vascular tissue within the bladder wall, increased oxidative stress, and eventually fibrosis.¹⁴ Because the urinary tract is the final exit route for these metabolites, the bladder lining sits in prolonged contact with them, especially in people using ketamine frequently. This is a major reason chronic recreational users develop ketamine-associated cystitis, a painful condition that can severely reduce bladder capacity. The fact that the kidney itself also metabolizes ketamine means the urinary tract gets a double hit: metabolites arriving from the liver via the blood and metabolites being generated locally in the kidney.

Chronic Use Changes the Equation

Repeated ketamine exposure can alter its own metabolism through enzyme induction. In patients who had been on long-term barbiturates, steady-state plasma levels of ketamine were significantly lower than in barbiturate-naive patients, because the barbiturates had ramped up the activity of the liver enzymes that break ketamine down.¹⁵ This principle extends to ketamine itself: repeated doses can induce the very enzymes responsible for its clearance, contributing to the tolerance that chronic users develop. Over time, a person may need higher doses to achieve the same blood levels, which in turn produces more metabolites and potentially more toxicity to the bladder and urinary tract.

Detection Windows in Urine

For forensic and clinical monitoring purposes, the different metabolites offer different detection windows. Using sensitive mass spectrometry, ketamine itself can be confirmed in urine for up to five days after use, norketamine for up to six days, and dehydronorketamine for up to ten days.¹⁶ DHNK’s longer detection window makes it particularly useful as a marker in drug testing scenarios where several days have passed since the last dose. Standard immunoassay drug panels used in emergency departments and workplaces do not typically screen for ketamine, so specific testing must usually be requested.

Drug Interactions at the Enzyme Level

Because ketamine relies heavily on CYP3A4 and to a lesser extent CYP2B6 and CYP2C9, anything that inhibits or accelerates these enzymes can alter ketamine’s metabolism. This is not just a theoretical concern in the context of anesthesia or pain management, where multiple drugs are routinely given together. In veterinary research comparing humans, horses, and dogs, both methadone and xylazine inhibited the in vitro metabolism of ketamine across all three species, meaning concurrent use of these analgesics could lead to higher ketamine concentrations and greater risk of side effects.¹⁷

The pattern of which specific CYP enzyme matters most varies across species. In human liver tissue, a CYP3A4 inhibitor was the most potent blocker of ketamine breakdown. In dog and horse tissue, inhibitors of CYP2A6 and CYP2C19 had the strongest effect.¹⁸ For human patients, the most clinically relevant interactions involve CYP3A4 inhibitors such as certain antifungals, macrolide antibiotics, and grapefruit juice, as well as CYP3A4 inducers like rifampin, carbamazepine, and St. John’s wort. An inhibitor will slow ketamine clearance and raise blood levels; an inducer will speed clearance and potentially reduce the drug’s effect.

Why Route of Administration Matters

How ketamine enters the body determines how much of it survives to reach the bloodstream intact. When given intravenously, the full dose enters circulation immediately and metabolism begins as the blood passes through the liver. When taken orally, however, ketamine must first pass through the gut wall and liver before reaching general circulation, a process called first-pass metabolism. This chews up a large fraction of the dose, which is why oral bioavailability of ketamine is estimated at only about 20 to 25 percent. The tradeoff is that oral dosing produces proportionally more norketamine and downstream metabolites relative to the parent drug. If those metabolites contribute meaningfully to the antidepressant effect, oral administration might paradoxically favor a better metabolite profile for mood disorders despite delivering less ketamine itself.

Intranasal administration, as with esketamine, falls somewhere in between: bioavailability is higher than oral but lower than intravenous, and the metabolite profile differs accordingly. Sublingual and intramuscular routes each have their own absorption characteristics, which is part of why the clinical experience of ketamine can vary so much depending on how it is given.

Species Differences in Ketamine Breakdown

Veterinary medicine uses ketamine far more widely than human medicine, and species differences in metabolism are real and sometimes dramatic. The CYP3A4 enzyme family and its orthologs are involved in ketamine breakdown across humans, horses, and dogs.¹⁷ But a general CYP inhibitor that nearly completely blocked ketamine metabolism in human and dog liver tissue did not have the same effect in horse tissue, suggesting that horses rely on additional or alternative enzymatic pathways not present in the other two species.¹⁸ These differences matter practically: a drug combination that is safe in a human patient could produce dangerously prolonged ketamine effects in a dog if the co-administered drug happens to block a CYP isoform the dog relies on more heavily.