Ketamine Pharmacology: Absorption, Distribution, Metabolism

Ketamine pharmacology describes how the drug is absorbed, distributed through the body, metabolized in the liver, and eliminated by the kidneys, and these pharmacokinetic properties explain why an IV infusion produces different effects than a nasal spray or an oral formulation. Because ketamine is a chiral molecule, its two enantiomers, S-ketamine and R-ketamine, differ in potency and receptor binding, which affects how prescribers dose and interpret clinical protocols. Ketamine is classified as an arylcyclohexylamine, with a molecular formula of C13H16ClNO and a molecular weight of 237.7 g/mol.

This article covers each phase of ketamine pharmacology in the order clinicians use it: absorption and bioavailability by route, distribution into the brain and body tissues, hepatic metabolism into norketamine and hydroxynorketamine, and renal elimination, along with the drug interactions and patient-specific factors that alter these processes. For a broader look at how ketamine produces its effects at the receptor level, see our guide on how ketamine works.

Enantiomers: S-Ketamine and R-Ketamine

Chirality and Clinical Significance

Ketamine is a chiral molecule, meaning it exists as two mirror-image forms called enantiomers, with a single chiral center at the C-2 carbon of its cyclohexanone ring. These two forms are S(+)-ketamine (esketamine) and R(-)-ketamine (arketamine). Standard pharmaceutical ketamine is a racemic mixture, containing equal proportions of both enantiomers. See our comparison of racemic ketamine and esketamine for how this distinction plays out in treatment decisions.

S-ketamine has approximately 3-4 times greater affinity for the NMDA receptor, a glutamate receptor in the brain involved in learning, memory, and the drug's dissociative effects, than R-ketamine does. It is the more potent anesthetic and analgesic enantiomer and produces more pronounced psychotomimetic and dissociative effects. S-ketamine is marketed as Spravato, an FDA-approved nasal spray for treatment-resistant depression. Our esketamine guide covers its approval, dosing, and clinical use in more detail.

R-ketamine has lower NMDA receptor affinity but has shown promising antidepressant effects in preclinical studies, potentially with fewer dissociative side effects. It may work through AMPA receptor-mediated mechanisms and BDNF signaling rather than through strong NMDA blockade, and several clinical trials of arketamine are currently underway.

Clinical Implications of Enantiomer Differences

Because S-ketamine is a more potent anesthetic, it can be used at lower doses for anesthesia. Its stronger psychotomimetic effects may be viewed as therapeutic, since some clinicians believe dissociation correlates with antidepressant response, or as undesirable, since some patients find the experience distressing.

Absorption and Bioavailability by Route

The bioavailability of ketamine, the fraction of an administered dose that reaches systemic circulation, varies widely by route of administration. This variation is one of the most clinically relevant aspects of ketamine pharmacology, since it determines how quickly a dose takes effect and how much of it reaches the brain.

Intravenous (IV)

IV bioavailability is 100% by definition. The standard antidepressant dose used in research and clinical infusion protocols is 0.5 mg/kg administered over 40 minutes, with peak plasma levels reached at the end of the infusion. IV remains the most studied route for psychiatric applications.

Intramuscular (IM)

According to StatPearls, a peer-reviewed clinical reference maintained by the National Library of Medicine, intramuscular ketamine has a bioavailability of approximately 93%. Onset is rapid, typically within 3-5 minutes, with peak plasma levels around 20 minutes. Because IM injection largely bypasses hepatic first-pass metabolism, it retains most of the administered dose.

Intranasal

Intranasal bioavailability ranges from about 25-50%, with variability tied to technique, nasal mucosa condition, and formulation. Spravato, the FDA-approved esketamine nasal spray for treatment-resistant depression, reports bioavailability of approximately 48%, with onset generally within 5-10 minutes.

Sublingual and Buccal

Sublingual and buccal bioavailability ranges from about 24-30%. Absorption through the oral mucosa partially bypasses hepatic first-pass metabolism, but any swallowed portion still undergoes significant liver processing, which lowers effective bioavailability. Onset is typically 15-30 minutes.

Oral

Oral bioavailability is approximately 16-24%, the lowest of the common routes, because extensive hepatic first-pass metabolism breaks down much of the dose before it reaches systemic circulation. This route does produce comparatively high levels of the active metabolite norketamine, which may contribute its own therapeutic effects.

Rectal

Rectal bioavailability is approximately 25-30%. This route is occasionally used in pediatric anesthesia but is rarely used for psychiatric applications. For a broader comparison of how these routes are used in practice, see our guide to ketamine treatment methods.

Distribution

Plasma Protein Binding

Ketamine binds moderately to plasma proteins, approximately 10-30%, primarily to albumin and alpha-1 acid glycoprotein. Because this binding is relatively low, a large fraction of circulating ketamine remains unbound and free to cross the blood-brain barrier.

Volume of Distribution

Ketamine has a large volume of distribution, approximately 3-5 L/kg, reflecting extensive movement into body tissues. Its high lipophilicity allows it to cross the blood-brain barrier quickly, which explains its fast onset of central nervous system effects.

Brain Penetration and Redistribution

Ketamine crosses the blood-brain barrier within minutes of IV administration due to its lipophilicity and small molecular size. It distributes first to highly perfused tissues, including the brain, heart, and lungs, before redistributing to muscle and fat. This redistribution out of the brain, not metabolism, is the main reason ketamine's acute dissociative effects end after 15-45 minutes following a standard infusion, even though the drug remains measurable in the blood for hours longer. Readers who want a closer look at how this affects the treatment experience can see our guide on understanding dissociation.

Metabolism

Hepatic Biotransformation

Ketamine undergoes extensive liver metabolism through the cytochrome P450 (CYP) enzyme system, a family of liver enzymes responsible for breaking down most prescription drugs. The primary pathway is N-demethylation to norketamine, mediated mainly by CYP2B6 and CYP3A4, with smaller contributions from CYP2C9, CYP2C19, and CYP2D6.

Norketamine

Norketamine is ketamine's major active metabolite. It retains roughly 20-30% of the parent compound's anesthetic potency and has a longer elimination half-life. Growing evidence suggests norketamine may contribute to ketamine's antidepressant effects, particularly after oral dosing, which produces proportionally higher norketamine levels due to first-pass metabolism.

Hydroxynorketamine and Further Metabolism

Norketamine is further metabolized into hydroxynorketamine (HNK) isomers, most notably (2R,6R)-HNK and (2S,6S)-HNK. According to a 2016 study by Zanos and colleagues published in Nature, (2R,6R)-HNK produced antidepressant-like effects in mice without blocking NMDA receptors or producing dissociative behavior. If this mechanism is confirmed in human research, it could support development of ketamine-derived antidepressants without the dissociative and abuse-related effects of the parent drug. HNK metabolites are eventually conjugated through glucuronidation and excreted. A related metabolite, dehydronorketamine (DHNK), forms through dehydrogenation of norketamine but appears to have minimal pharmacological activity.

Elimination

Half-Life

Ketamine's elimination half-life is approximately 2-3 hours in adults. Norketamine's half-life is slightly longer, around 4-6 hours, and the HNK metabolites persist even further, with half-lives extending beyond 12 hours.

Elimination half-life is not the same as duration of clinical effects. Dissociative and psychoactive effects typically resolve within 1-2 hours after an IV infusion, driven mainly by redistribution rather than elimination. Antidepressant effects, by contrast, can persist for days to weeks after the drug has cleared the body, because they are thought to depend on structural synaptic changes rather than ongoing receptor occupancy. Our article on synaptogenesis covers this mechanism in more detail.

Renal Excretion

Approximately 90% of a ketamine dose is eventually excreted through the kidneys, mostly as hydroxylated and conjugated metabolites. Less than 4% is excreted as unchanged ketamine, and the remaining fraction leaves the body through feces.

Factors That Affect Elimination

Several patient and drug factors influence how quickly ketamine clears the body:

• Age: elderly patients generally metabolize ketamine more slowly and may need dose adjustments
• Hepatic function: liver disease can significantly impair ketamine metabolism, since the drug is almost entirely cleared by the liver
• CYP enzyme polymorphisms: genetic variation in CYP2B6 and CYP3A4 contributes to differences in drug levels between patients
• Concurrent medications: CYP3A4 inhibitors such as ketoconazole, clarithromycin, and grapefruit juice can slow ketamine metabolism, while inducers such as rifampin and carbamazepine can speed it up

Drug Interactions

Pharmacokinetic Interactions

Medications that inhibit or induce CYP2B6 and CYP3A4 can change ketamine plasma levels. Clinically relevant examples include:

• CYP3A4 inhibitors, such as ketoconazole, itraconazole, and ritonavir, may increase ketamine levels
• CYP3A4 inducers, such as rifampin, phenytoin, and carbamazepine, may decrease ketamine levels
• CYP2B6 inhibitors, such as ticlopidine and clopidogrel, may increase ketamine levels

Pharmacodynamic Interactions

Ketamine's effects can also be altered by drugs that do not change its blood levels but interact with its mechanism of action:

• Benzodiazepines may blunt ketamine's psychoactive effects, and some studies report lower antidepressant response rates when they are taken concurrently
• Lamotrigine, a glutamate release inhibitor, has been studied as a pretreatment to reduce ketamine's psychotomimetic effects, though results on antidepressant efficacy are mixed
• Opioids are sometimes combined with ketamine for pain management, and the two drug classes may produce synergistic analgesic effects
• MAOIs carry a theoretical risk of hypertensive crisis when combined with ketamine, though clinical data are limited; most clinicians use caution with this combination

Clinical Pharmacology Considerations

Repeated Dosing and Tolerance

Some degree of pharmacodynamic tolerance develops to ketamine's dissociative and psychotomimetic effects with repeated administration, and many patients report that dissociation feels less intense with successive treatments. Whether tolerance also develops to the antidepressant effects is still under investigation.

Pharmacogenomics

Genetic variation in metabolic enzymes, NMDA receptor subunitsBDNF (notably the Val66Met polymorphism), and other targets may influence how individual patients respond to ketamine. Pharmacogenomic testing is not yet standard practice in ketamine therapy, but it represents a promising direction for more personalized dosing.