Mechanism of action
Chloral hydrate is a prodrug that is rapidly metabolized in the liver and erythrocytes by alcohol dehydrogenase to its active metabolite, trichloroethanol. Trichloroethanol is responsible for the sedative-hypnotic effects.
The exact mechanism involves enhancement of GABA_A receptor-mediated chloride conductance, similar to barbiturates and benzodiazepines, leading to CNS depression. Trichloroethanol potentiates inhibitory neurotransmission, resulting in sedation, hypnosis, and anxiolysis.
Unlike benzodiazepines, chloral hydrate does not have a specific receptor antagonist, making reversal of overdose more challenging. The drug produces dose-dependent CNS depression ranging from mild sedation to deep sleep.
- Chloral hydrate is converted to active metabolite trichloroethanol by alcohol dehydrogenase
- Primarily used for procedural sedation in children undergoing non-painful imaging studies
- Enhances GABA_A receptor-mediated chloride conductance, causing CNS depression
- Trichloroacetic acid (inactive metabolite) displaces warfarin from protein binding sites
- Trichloroethanol half-life is prolonged in neonates (up to 40 hours) due to immature metabolism
- Combination with alcohol produces synergistic dangerous CNS depression (Mickey Finn)
- Schedule IV controlled substance with potential for dependence and alcohol-like withdrawal
- No specific antidote for overdose; management is supportive care
Indications
Chloral hydrate has limited but specific clinical uses:
- Procedural sedation in pediatrics: Most common current use, particularly for non-painful procedures such as MRI, CT scans, EEG, echocardiography, and auditory brainstem response testing in infants and young children
- Short-term treatment of insomnia: Rarely used today due to availability of safer alternatives
- Sedation before dental procedures in anxious pediatric patients
- Preoperative sedation in select cases
Choral hydrate has largely been replaced by newer agents such as benzodiazepines, dexmedetomidine, and propofol in many settings due to safety concerns and variable efficacy. However, it remains useful for non-invasive pediatric imaging because it produces sedation without significant respiratory depression at therapeutic doses and does not require IV access.
Pharmacokinetics
Absorption: Rapidly absorbed from the gastrointestinal tract after oral administration. Peak plasma concentrations of trichloroethanol occur within 30-60 minutes.
Distribution: Both chloral hydrate and trichloroethanol are widely distributed. Trichloroethanol is highly protein-bound (approximately 35-40%). The drug crosses the placenta and is excreted in breast milk.
Metabolism: Chloral hydrate undergoes rapid first-pass metabolism by alcohol dehydrogenase to trichloroethanol (active metabolite) in the liver and red blood cells. Trichloroethanol is further metabolized by glucuronidation to trichloroacetic acid (inactive) and urochloralic acid.
Elimination: Primarily renal excretion of metabolites. The half-life of trichloroethanol is 8-12 hours in adults but may be prolonged in neonates (up to 40 hours) due to immature hepatic enzyme systems. In adults, trichloroacetic acid has a much longer half-life of several days and can accumulate with repeated dosing.
Onset and duration: Sedation typically begins within 30-60 minutes after oral administration and lasts 4-8 hours, though children may sleep longer.
Adverse effects
Common adverse effects include:
- Gastrointestinal: Nausea, vomiting, abdominal pain, diarrhea; gastric irritation (can be minimized by diluting and administering with food or milk)
- CNS: Paradoxical agitation or hyperactivity (particularly in children), confusion, ataxia, hangover effect, dizziness
- Respiratory: Respiratory depression (dose-dependent), airway obstruction
- Dermatologic: Rash, urticaria
Serious adverse effects:
- Cardiovascular: Cardiac arrhythmias, including ventricular arrhythmias; hypotension
- Hepatotoxicity: Rare cases of hepatic dysfunction with prolonged use
- Dependence and tolerance: Physical and psychological dependence can develop with chronic use; withdrawal syndrome similar to alcohol withdrawal
- Sudden death: Rare reports, particularly with higher doses or in combination with other CNS depressants
Overdose: Presents with severe CNS depression, respiratory depression, hypotension, hypothermia, and cardiac arrhythmias. Gastric necrosis and esophageal stricture can occur with large ingestions. Management is supportive as there is no specific antidote.
Contraindications and warnings
Absolute contraindications:
- Severe hepatic impairment
- Severe renal impairment
- Known hypersensitivity to chloral hydrate
- Gastritis or peptic ulcer disease (oral irritant)
- Marked cardiac disease
Warnings and precautions:
- Respiratory disease: Use with extreme caution in patients with sleep apnea, chronic obstructive pulmonary disease, or other respiratory compromise
- Porphyria: Chloral hydrate may precipitate acute porphyric attacks
- Pregnancy: Category C. Crosses the placenta; use only if benefit outweighs risk. Neonatal withdrawal has been reported
- Lactation: Excreted in breast milk and may cause sedation in nursing infants
- Elderly patients: Increased sensitivity to CNS depressant effects
- Abuse potential: Schedule IV controlled substance in the United States due to potential for dependence
- Procedure failure: Sedation failure rate of 5-10% in pediatric procedural sedation; some children may experience paradoxical agitation
Drug displacement: Chloral hydrate and its metabolites can displace other highly protein-bound drugs, increasing their free concentrations.
Drug interactions
CNS depressants: Additive sedative effects with alcohol, benzodiazepines, opioids, barbiturates, antihistamines, and other sedating medications. This combination increases risk of respiratory depression.
Warfarin: The highly protein-bound inactive metabolite, trichloroacetic acid, displaces warfarin from plasma proteins, transiently increasing free warfarin concentration and enhancing anticoagulant effect with increased bleeding risk. INR should be monitored closely.
Furosemide: When oral or rectal chloral hydrate is given in combination with IV furosemide, a syndrome of flushing, sweating, tachycardia, and blood pressure changes can occur due to displacement of thyroid hormone and subsequent hypermetabolic state.
Phenytoin: Chloral hydrate may transiently increase phenytoin levels through protein binding displacement.
Alcohol: The combination produces a synergistic CNS depressant effect historically known as a "Mickey Finn" or "knockout drops." This combination is extremely dangerous and can be fatal.
Other interactions: May potentiate effects of other protein-bound drugs through displacement. Concurrent use with other hepatotoxic drugs may increase risk of liver injury.
Clinical pearls
- Chloral hydrate is one of the oldest synthetic sedatives still in clinical use, dating back to 1869, though its use has dramatically declined.
- The drug is particularly useful for pediatric neuroimaging because it provides adequate sedation without requiring IV access and has minimal effects on brain metabolism or cerebral blood flow at standard doses.
- Sedation failure occurs in approximately 5-10% of children; younger age and previous failed sedation are risk factors. Have a backup sedation plan ready.
- The "Mickey Finn" phenomenon refers to the historical use of chloral hydrate mixed with alcohol to incapacitate victims, producing profound and dangerous sedation.
- Chloral hydrate can cause false-positive urine catecholamine tests and interfere with fluorometric urine tests, which is relevant in patients being evaluated for pheochromocytoma.
- Tolerance develops rapidly with repeated dosing, and withdrawal symptoms resemble those of alcohol withdrawal, including tremors, anxiety, seizures, and delirium.
- The active metabolite trichloroethanol has a significantly prolonged half-life in neonates, making this population particularly vulnerable to accumulation and prolonged sedation.
- Chloral hydrate should be diluted and given with food or liquid to minimize gastric irritation, which can be severe and lead to nausea and vomiting.
- Modern alternatives such as dexmedetomidine, midazolam, or propofol have largely replaced chloral hydrate in many institutions due to better safety profiles and more predictable effects.
Practice questions
1. A 2-year-old boy is brought to the radiology department for a non-contrast brain MRI to evaluate developmental delay. The physician orders oral chloral hydrate for procedural sedation. Thirty minutes after administration, the child becomes adequately sedated, and the MRI is completed successfully. Which of the following best describes the mechanism by which this medication produces its sedative effect?
2. A 68-year-old man with atrial fibrillation on warfarin therapy is given chloral hydrate for insomnia during a hospital admission. Two days later, his INR increases from 2.5 to 5.8 without any change in warfarin dose. He has no signs of bleeding. Which of the following best explains this laboratory finding?
3. A 6-month-old girl is brought to the emergency department with lethargy and respiratory depression. Her parents report she was given 'sleep medicine' 3 hours ago for an outpatient MRI but became increasingly difficult to arouse. Vital signs show respiratory rate 8/min, heart rate 90/min, and oxygen saturation 88% on room air. Which of the following is the most appropriate immediate management?
Frequently asked questions
How does chloral hydrate work?
Chloral hydrate is a prodrug that is rapidly converted to trichloroethanol, its active metabolite, by alcohol dehydrogenase in the liver and red blood cells. Trichloroethanol enhances GABA_A receptor-mediated chloride conductance in the central nervous system, similar to benzodiazepines and barbiturates, leading to dose-dependent CNS depression. This mechanism produces sedation, hypnosis, and anxiolysis without significant analgesic properties.
What are the side effects of chloral hydrate?
Common side effects include gastrointestinal upset (nausea, vomiting, abdominal pain), paradoxical agitation especially in children, hangover effect, and dizziness. Serious adverse effects include respiratory depression, cardiac arrhythmias, hypotension, and potential for physical dependence with chronic use. Gastric irritation can be severe, and the drug can cause withdrawal symptoms similar to alcohol withdrawal syndrome if discontinued abruptly after prolonged use. Sedation failure occurs in five to ten percent of pediatric patients.
Is chloral hydrate a controlled substance?
Yes, chloral hydrate is classified as a Schedule IV controlled substance in the United States under the Controlled Substances Act. This classification reflects its potential for abuse, physical dependence, and psychological dependence, though the risk is considered lower than Schedule II or III substances. Chronic use can lead to tolerance and a withdrawal syndrome resembling alcohol withdrawal, including tremors, anxiety, and potentially seizures.
What is chloral hydrate used for in children?
Chloral hydrate is primarily used for procedural sedation in infants and young children undergoing non-painful diagnostic procedures such as MRI, CT scans, EEG, echocardiography, and auditory brainstem response testing. It is favored in this setting because it provides adequate sedation without requiring intravenous access, produces minimal respiratory depression at therapeutic doses, and does not significantly affect brain metabolism. The typical onset is thirty to sixty minutes after oral administration, with sedation lasting four to eight hours.
Chloral hydrate vs midazolam — what is the difference?
Chloral hydrate is an older sedative-hypnotic prodrug converted to trichloroethanol, administered orally for pediatric procedural sedation, with no specific reversal agent and a slower onset of thirty to sixty minutes. Midazolam is a benzodiazepine that can be given orally, intranasally, or intravenously, has a faster onset (especially IV), and can be reversed with flumazenil. Midazolam provides more predictable sedation and has a better safety profile, while chloral hydrate does not require IV access but has a higher failure rate and potential for gastric irritation.
Conditions it is used for
Related drugs
Sources and further reading
DailyMed (FDA labels)StatPearls (NCBI Bookshelf)MedlinePlusWHO ATC/DDD index: N05CC01
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.