Pharmacokinetics of Ketamine: Absorption, Distribution, Metabolism, and Elimination

The pharmacokinetics of ketamine describe how the medication is absorbed, distributed, metabolized, and eliminated. Route of administration changes how much ketamine reaches the bloodstream, how quickly effects begin, and the relative exposure to metabolites such as norketamine. These differences help explain why dosing, monitoring, and timing of assessment should be determined by a qualified clinician.

Ketamine is a phencyclidine derivative with a molecular weight of 237.7 daltons. It is moderately lipophilic, allowing rapid passage across the blood-brain barrier. Racemic ketamine contains two enantiomers, S(+)-ketamine, also called esketamine, and R(-)-ketamine, also called arketamine. S-ketamine has greater NMDA receptor affinity than R-ketamine, while the two forms also differ in clearance and pharmacodynamic effects.

Absorption and bioavailability by route

Route is one of the main drivers of ketamine pharmacokinetics. Bioavailability means the proportion of a dose that reaches systemic circulation, and Tmax is the time at which the highest plasma concentration is reached.

Intravenous administration

IV ketamine has 100 percent bioavailability by definition and serves as the reference route for pharmacokinetic comparisons. With a 0.5 mg/kg infusion over 40 minutes, reported peak plasma concentrations are often about 150-250 ng/mL. Central nervous system effects can begin within minutes of starting an infusion.

Intramuscular and subcutaneous administration

Intramuscular ketamine has reported bioavailability of about 93 percent, with Tmax often around 15-30 minutes. Subcutaneous administration has been estimated to have bioavailability near 90 percent, with absorption that may be slightly slower than IM administration. Local blood flow, injection site, and patient factors can affect exposure.

Intranasal administration

Intranasal ketamine has reported bioavailability of roughly 25-50 percent, with substantial variation between patients. Nasal congestion, mucosal blood flow, technique, and formulation can affect absorption. Tmax is commonly reported at 20-40 minutes. The FDA prescribing information for Spravato describes esketamine nasal spray, a standardized intranasal formulation, and its pharmacokinetic profile.

Sublingual and oral administration

Sublingual ketamine has reported bioavailability of about 25-30 percent. Oral bioavailability is lower, often reported around 16-24 percent, because a significant share is metabolized during first pass through the liver. Oral dosing therefore produces a higher norketamine-to-ketamine ratio than IV dosing. Readers considering non-IV routes can review this site's ketamine troche dosage guide for patient-focused context.

Distribution: why effects can fade before the drug is fully eliminated

Ketamine distributes rapidly into highly perfused tissues, including the brain. Its steady-state volume of distribution is commonly reported at approximately 3-5 L/kg, which reflects extensive tissue uptake. Plasma protein binding is relatively low, reported at approximately 12-47 percent, primarily involving alpha-1-acid glycoprotein and albumin.

Ketamine follows a two- or three-compartment model. After a single bolus, rapid redistribution from brain tissue to peripheral tissues contributes to the shorter duration of acute effects. That is different from terminal elimination half-life, which describes the slower final phase of decline in plasma concentration. Clinical effects, impairment, and recovery should not be inferred from half-life alone. See how long ketamine effects last for a related discussion of the patient experience and monitoring considerations.

Metabolism: ketamine, norketamine, and hydroxynorketamine

The liver is the primary site of ketamine clearance. CYP3A4 and CYP2B6 are important cytochrome P450 enzymes involved in N-demethylation of ketamine to norketamine. Norketamine has pharmacological activity and may contribute to therapeutic and adverse effects, particularly after oral administration.

Norketamine is further metabolized to hydroxynorketamine, or HNK, isomers and then undergoes glucuronide conjugation before renal excretion. The (2R,6R)-hydroxynorketamine metabolite has been studied for possible antidepressant mechanisms in preclinical research. Human clinical findings have not established that HNK alone explains ketamine's antidepressant effects.

According to the National Library of Medicine's PubChem record for ketamine, ketamine has a molecular weight of 237.7 g/mol. For a clinical overview of pharmacokinetic and pharmacodynamic findings across ketamine uses, see the peer-reviewed review indexed by PubMed.

Elimination half-life and factors that can change exposure

Racemic ketamine has a terminal elimination half-life of approximately 2-3 hours in healthy adults. The earlier distribution phase is much shorter, commonly about 10-15 minutes, and is often more relevant to the decline of acute effects after a single bolus. S-ketamine is generally cleared somewhat faster than R-ketamine. Norketamine has been reported to have an elimination half-life of approximately 5-8 hours, while some HNK metabolites may persist longer.

Ketamine has high hepatic clearance, reported at roughly 12-20 mL/kg/min. Reduced hepatic blood flow, hepatic impairment, age, body composition, CYP2B6 or CYP3A4 variation, and concurrent medications can all affect exposure. CYP3A4 inhibitors or inducers may require clinical review because they can alter metabolism. A clinician should also consider whether weight-based dosing needs adjustment for a patient with obesity, since distribution into adipose tissue does not necessarily correspond to proportional clearance.

These are pharmacokinetic considerations, not a dosing formula. Prescribers should individualize route, dose, assessment timing, and monitoring. For related practical context, read ketamine drug interactions and monitoring and assessment during ketamine treatment.

What this means for low-dose ketamine care

Pharmacokinetic data can help patients understand why treatment experiences differ, but they cannot predict an individual response with certainty. A lower-bioavailability route may have a slower onset and more metabolite exposure, while IV administration gives the most direct control over delivered dose. The appropriate approach depends on the clinical indication, medical history, medications, and the treatment setting.

People with possible liver disease, cardiovascular concerns, or medication changes should raise those details before treatment. This site also covers cardiovascular monitoring with ketamine and common ketamine side effects.