Mechanism of action
Ketamine acts primarily as a noncompetitive antagonist at the N-methyl-D-aspartate (NMDA) subtype of glutamate receptors in the central nervous system. By blocking the NMDA receptor channel in a use-dependent manner, ketamine interrupts excitatory neurotransmission and produces a dissociative state in which the patient appears awake but is disconnected from the environment and unresponsive to pain.
Ketamine also interacts with several other receptor systems, contributing to its complex pharmacologic profile:
- Opioid receptors: weak agonist activity at μ and κ receptors, contributing to analgesia
- Monoaminergic systems: inhibits reuptake of norepinephrine, dopamine, and serotonin
- Muscarinic and nicotinic receptors: modest anticholinergic effects
- Voltage-gated sodium and calcium channels: local anesthetic properties at higher concentrations
The antidepressant effect of ketamine, particularly its rapid onset within hours, is thought to result from NMDA antagonism leading to a surge in glutamate release, activation of AMPA receptors, and subsequent upregulation of brain-derived neurotrophic factor (BDNF) and synaptogenesis in prefrontal cortical circuits.
Unlike most anesthetics, ketamine produces sympathomimetic cardiovascular effects by inhibiting catecholamine reuptake centrally and peripherally, leading to increased heart rate, blood pressure, and cardiac output. This effect can be beneficial in hemodynamically unstable patients but problematic in those with coronary artery disease or hypertension.
- Ketamine is a noncompetitive NMDA receptor antagonist producing dissociative anesthesia with preserved airway reflexes and spontaneous ventilation.
- Contraindicated in elevated intracranial pressure due to increased cerebral blood flow and ICP.
- Causes sympathomimetic cardiovascular effects: increased HR, BP, and cardiac output—useful in shock but problematic in CAD.
- Emergence reactions (hallucinations, agitation, delirium) occur in 10–30% and are reduced by benzodiazepine coadministration.
- Increases salivation; pretreatment with anticholinergic (atropine, glycopyrrolate) is common.
- Produces bronchodilation, making it preferred in patients with reactive airway disease or asthma.
- Rapid-acting antidepressant at subanesthetic doses (0.5 mg/kg IV), with onset in hours; used for treatment-resistant depression.
- Schedule III controlled substance with abuse potential; chronic use causes ketamine cystitis and cognitive impairment.
Indications
Ketamine is approved and used for multiple clinical scenarios:
Anesthesia and procedural sedation
- Induction and maintenance of general anesthesia, particularly in hemodynamically unstable patients, trauma, or hemorrhagic shock
- Procedural sedation in emergency departments and for painful procedures (fracture reduction, wound debridement, burn dressing changes)
- Pediatric sedation for imaging, laceration repair, and minor procedures
Analgesia
- Acute pain management, often as an adjunct to opioids to reduce total opioid requirements
- Chronic pain syndromes, including complex regional pain syndrome (CRPS) and neuropathic pain
- Palliative care analgesia for opioid-refractory pain
Psychiatric indications
- Treatment-resistant depression (intranasal esketamine is FDA-approved; intravenous racemic ketamine is used off-label)
- Rapid reduction of acute suicidal ideation in controlled settings
Special populations
- Preferred anesthetic in patients with reactive airway disease (bronchodilator properties)
- Useful in mass casualty and battlefield settings due to preserved respiratory drive and ease of intramuscular administration
Ketamine is a Schedule III controlled substance in the United States due to potential for abuse and psychological dependence.
Pharmacokinetics
Ketamine exhibits rapid onset and relatively short duration of action, with pharmacokinetics that vary by route of administration.
| Parameter | Value |
|---|---|
| Bioavailability | IV 100%, IM 93%, intranasal ~45%, oral ~20% (high first-pass metabolism) |
| Onset of action | IV: 30 seconds; IM: 3–5 minutes; intranasal: 5–10 minutes |
| Duration | IV: 5–15 minutes (anesthesia); IM: 15–30 minutes |
| Distribution | Vd ~3 L/kg; highly lipophilic, rapidly crosses blood-brain barrier |
| Protein binding | ~25–50% |
| Metabolism | Hepatic via CYP3A4, CYP2B6, and CYP2C9 to norketamine (active metabolite with 20–30% potency) |
| Elimination half-life | 2–3 hours (ketamine); 12 hours (norketamine) |
| Excretion | Renal (90% as metabolites) |
Ketamine is formulated as a racemic mixture of S(+) and R(−) enantiomers. Esketamine (S-ketamine) is the more potent enantiomer with approximately twice the affinity for NMDA receptors and is available as an intranasal formulation for depression.
The hepatic first-pass effect significantly reduces oral bioavailability, making parenteral and intranasal routes preferred for acute use. Dosing adjustments may be needed in hepatic impairment.
Adverse effects
Ketamine produces a distinctive adverse effect profile related to its dissociative and sympathomimetic properties.
Common and characteristic
- Emergence reactions (10–30%): vivid dreams, hallucinations, delirium, agitation during recovery; reduced by benzodiazepine premedication and minimizing stimulation during emergence
- Dissociation: out-of-body sensations, depersonalization, derealization
- Nystagmus and diplopia: common during dissociative state
- Increased salivation and oral secretions: pretreatment with anticholinergic (glycopyrrolate, atropine) often used
- Nausea and vomiting: particularly during emergence
Cardiovascular
- Hypertension and tachycardia: due to sympathomimetic effects; problematic in uncontrolled hypertension, coronary artery disease, aortic dissection
- Arrhythmias: rare but possible in susceptible patients
Neurologic
- Increased intracranial pressure (ICP): due to increased cerebral blood flow; contraindication in head trauma or mass lesions
- Increased intraocular pressure: avoid in acute globe injury
- Myoclonus and involuntary muscle movements
Respiratory
- Laryngospasm: rare but more common in children under 3 months or with active upper respiratory infection
- Respiratory depression: uncommon at standard doses but can occur with rapid IV push or in combination with other sedatives
- Bronchodilation: generally beneficial in asthma or bronchospasm
Psychiatric and chronic use
- Tolerance and dependence: with repeated or recreational use
- Ketamine cystitis: ulcerative cystitis with chronic use, presenting as dysuria, hematuria, and frequency
- Cognitive impairment: memory deficits and executive dysfunction with chronic heavy use
- Hepatotoxicity: rare but reported with chronic abuse
Other
- Transient elevation in liver enzymes
- Rash at injection site or generalized
Contraindications and warnings
Absolute contraindications
- Conditions with elevated intracranial pressure (head trauma, intracranial mass, stroke, hydrocephalus): ketamine increases ICP and cerebral blood flow
- Severe uncontrolled hypertension or acute coronary syndrome: sympathomimetic effects worsen these conditions
- Known hypersensitivity to ketamine
Relative contraindications and precautions
- Coronary artery disease, heart failure, or arrhythmias: use with caution due to increased myocardial oxygen demand
- Acute globe injury or elevated intraocular pressure: ketamine raises IOP
- Severe hepatic impairment: reduced metabolism and clearance
- Active psychosis or schizophrenia: may exacerbate symptoms (though evidence is mixed for supervised psychiatric use)
- Thyroid dysfunction: particularly hyperthyroidism (potentiates sympathomimetic effects)
- Porphyria: theoretical risk of precipitating acute attack
- Age under 3 months: higher risk of airway complications, including laryngospasm
Pregnancy and lactation
- Pregnancy category: not formally assigned under current FDA labeling; animal studies show no teratogenicity but use only when benefit outweighs risk
- Crosses placenta; used in obstetric anesthesia in resource-limited settings but not first-line in developed countries
- Lactation: excreted in breast milk; short-term procedural use generally considered compatible with brief interruption of breastfeeding
Black box warnings
- Esketamine nasal spray carries warnings for sedation, dissociation, abuse potential, and suicidal thoughts and behaviors; requires Risk Evaluation and Mitigation Strategy (REMS) program with administration only in certified healthcare settings and monitored observation for at least 2 hours after each administration to ensure safety.
Drug interactions
Ketamine interacts with multiple drug classes through pharmacodynamic and pharmacokinetic mechanisms.
CNS depressants
- Benzodiazepines, opioids, barbiturates, propofol, alcohol: additive sedation and respiratory depression; dose reduction often needed
- Benzodiazepines (midazolam, lorazepam) are intentionally combined to reduce emergence reactions
Sympathomimetic agents
- Cocaine, amphetamines, decongestants, MAO inhibitors: additive sympathomimetic effects; risk of severe hypertension, tachycardia, and arrhythmias
- Caution with vasopressors and thyroid hormones
CYP450 interactions
- CYP3A4 inhibitors (ketoconazole, erythromycin, grapefruit juice): may increase ketamine levels and prolong effects
- CYP3A4 inducers (rifampin, phenytoin, carbamazepine): may reduce ketamine efficacy
- CYP2B6 substrates: potential competitive inhibition
Cardiovascular agents
- Beta-blockers: may unmask direct myocardial depressant effects of ketamine if sympathetic tone is blocked
- Calcium channel blockers and ACE inhibitors: interactions generally minor but monitor blood pressure
Other
- Nondepolarizing muscle relaxants: ketamine may prolong neuromuscular blockade
- Theophylline: case reports of seizures when combined with ketamine
- St. John's wort: CYP3A4 induction may reduce ketamine levels
Alcohol
- Acute alcohol intoxication increases risk of respiratory depression and aspiration; chronic use may alter ketamine metabolism
Clinical pearls
- Unique among general anesthetics, ketamine preserves airway reflexes, spontaneous ventilation, and hemodynamic stability, making it ideal for use in austere environments, unstable trauma patients, and procedural sedation without airway equipment.
- Emergence reactions are the most common limiting adverse effect; they are dose-dependent and can be reduced by coadministering a benzodiazepine, minimizing stimulation during recovery, and using lower doses for analgesia rather than anesthesia.
- Ketamine is contraindicated in elevated ICP because it increases cerebral blood flow and metabolic rate, raising intracranial pressure—this is a high-yield exam distinction from other anesthetics.
- The antidepressant effect occurs at subanesthetic doses (0.5 mg/kg IV) and has a rapid onset within hours, unlike traditional antidepressants that take weeks; effects last days to weeks, and repeated dosing or maintenance therapy is often required.
- Pretreatment with an anticholinergic (atropine or glycopyrrolate) reduces hypersalivation, which can be problematic especially in pediatric patients or those undergoing airway procedures.
- Ketamine causes bronchodilation and is particularly useful in patients with asthma or bronchospasm, unlike propofol or thiopental which can trigger bronchoconstriction.
- Ketamine cystitis is a debilitating complication of chronic recreational use, presenting with severe lower urinary tract symptoms, small bladder capacity, and sometimes irreversible damage requiring cystectomy.
- In mass casualty or battlefield settings, ketamine can be administered intramuscularly (4–5 mg/kg) without IV access, providing effective analgesia and sedation while maintaining respiratory drive—this is a key advantage over opioids or other anesthetics.
- The dissociative state produced by ketamine is characterized by catalepsy, amnesia, and analgesia; patients may appear awake with open eyes but are unresponsive to verbal commands and pain.
- Drug interactions with sympathomimetics (cocaine, amphetamines) or MAO inhibitors can cause life-threatening hypertensive crises and should be carefully screened.
- For procedural sedation, typical IV dosing is 1–2 mg/kg; for analgesia alone, lower doses (0.1–0.3 mg/kg) provide pain relief without full dissociation.
- Ketamine is increasingly used as an opioid-sparing analgesic in multimodal pain regimens, reducing total opioid consumption and associated side effects in postoperative and trauma patients.
Ketamine preserves the ABCs: Airway reflexes, Blood pressure (increases), Catecholamine surge (sympathomimetic).
Practice questions
1. A 28-year-old man is brought to the emergency department after a motor vehicle collision with hemodynamic instability (BP 85/50 mmHg, HR 120/min). He has multiple long bone fractures requiring emergent orthopedic surgery. The anesthesiologist selects ketamine for induction. Which of the following properties makes ketamine the preferred agent in this scenario?
2. A 45-year-old woman with treatment-resistant major depression receives intravenous ketamine infusion at a psychiatric clinic. Two hours after the infusion, she reports feeling significantly less depressed and hopeless. Which of the following best explains the rapid onset of ketamine's antidepressant effect?
3. A 6-year-old boy requires procedural sedation in the emergency department for closed reduction of a forearm fracture. He is given intravenous ketamine 1.5 mg/kg. Ten minutes into the procedure, the physician notices the child has copious oral secretions and increased salivation. Which of the following medications should have been administered prior to ketamine to prevent this complication?
Frequently asked questions
How does ketamine work?
Ketamine works as a noncompetitive antagonist at NMDA (N-methyl-D-aspartate) glutamate receptors in the central nervous system. By blocking these receptors, it interrupts excitatory neurotransmission and produces a dissociative anesthetic state characterized by analgesia, amnesia, and catalepsy while preserving airway reflexes and spontaneous breathing. It also has sympathomimetic effects by inhibiting catecholamine reuptake, leading to increased heart rate and blood pressure. The rapid antidepressant effect is thought to result from downstream activation of AMPA receptors and synaptogenesis.
What are the side effects of ketamine?
Common side effects include emergence reactions (hallucinations, vivid dreams, agitation in 10–30% of patients), dissociation, nystagmus, increased salivation, nausea, and vomiting. Cardiovascular effects include hypertension and tachycardia due to sympathomimetic activity. Ketamine increases intracranial and intraocular pressure. Rare but serious effects include laryngospasm (especially in young children) and respiratory depression with rapid IV administration. Chronic use leads to ketamine cystitis (painful bladder syndrome), cognitive impairment, tolerance, and dependence.
Is ketamine a controlled substance?
Yes, ketamine is a Schedule III controlled substance in the United States under the Controlled Substances Act. This classification reflects its accepted medical uses but also its potential for moderate to low physical dependence and psychological dependence. Ketamine has become a drug of abuse recreationally due to its dissociative and hallucinogenic effects. The FDA-approved esketamine nasal spray for treatment-resistant depression is subject to a Risk Evaluation and Mitigation Strategy (REMS) requiring supervised administration in certified healthcare settings.
Ketamine vs propofol — what is the difference?
Ketamine and propofol are both used for anesthesia and sedation but differ significantly in mechanism and effects. Ketamine is an NMDA antagonist that preserves airway reflexes, spontaneous breathing, and increases blood pressure (sympathomimetic), making it ideal for unstable patients. Propofol is a GABA agonist that causes respiratory depression, loss of airway reflexes, and hypotension, requiring airway management. Ketamine causes dissociation and emergence reactions; propofol provides smooth induction and emergence. Ketamine is contraindicated in elevated ICP while propofol reduces ICP. Propofol has antiemetic properties whereas ketamine increases nausea.
Can ketamine be used for depression?
Yes, ketamine has emerged as a rapid-acting treatment for treatment-resistant depression and acute suicidal ideation. At subanesthetic doses (typically 0.5 mg/kg IV over 40 minutes), ketamine produces antidepressant effects within hours, unlike traditional antidepressants that take weeks. The FDA has approved esketamine (S-ketamine) nasal spray for treatment-resistant depression, administered under medical supervision due to dissociation and abuse risks. Racemic ketamine IV is used off-label. Effects typically last days to weeks, requiring repeat dosing or maintenance protocols. The mechanism involves NMDA antagonism leading to increased synaptic plasticity and BDNF expression.
Conditions it is used for
Related drugs
Sources and further reading
DailyMed (FDA labels)StatPearls (NCBI Bookshelf)MedlinePlusWHO ATC/DDD index: N01AX03
Educational content for medical students, prepared by the WhiteCoat Pro editorial team with AI assistance; every page is reviewed by a second AI model for factual errors. It is not medical advice and does not replace the product label, local guidelines or a clinician.