Chloral hydrate is a sedative-hypnotic drug, one of the oldest synthetic sleep aids ever used in medicine. First created in a laboratory in 1832, it was the first true sleeping pill, predating barbiturates by decades. It works by depressing the central nervous system to produce drowsiness and sleep, and it remains classified as a Schedule IV controlled substance in the United States. Though largely replaced by newer sedatives, it still sees limited use today, primarily to sedate young children before medical procedures.
How Chloral Hydrate Works in the Body
Once swallowed, chloral hydrate is rapidly broken down by enzymes in the liver into its active form, a compound called trichloroethanol. This metabolite is what actually produces the sedative effect. It enhances the activity of the brain’s main calming chemical system (the same one targeted by drugs like benzodiazepines), slowing neural activity and promoting sleep. The body then processes trichloroethanol further, tagging it for removal through the kidneys.
This rapid conversion means the drug takes effect quickly. In studies of infants, chloral hydrate induced sleep within 10 minutes in half the subjects. Its effects typically last several hours, which makes it useful for keeping a child still during a 30- to 60-minute imaging scan but also means the sedation lingers well after a procedure ends.
A Brief History
German chemist Justus von Liebig first synthesized chloral hydrate in 1832, but no one tested it as a sedative until 1869, when Berlin pharmacologist Oskar Liebreich recognized its potential. It quickly became a preferred alternative to morphine and plant-based sedatives because it could be taken by mouth at home, eliminating the need for injections or confinement in a hospital. For decades it was the go-to sleeping aid before barbiturates arrived in the early 1900s.
Chloral hydrate also earned a darker reputation. Mixed with alcohol, it became the infamous “Mickey Finn” cocktail, a drink secretly spiked to knock someone unconscious. The combination is dangerously additive: alcohol increases the concentration of the drug in the bloodstream, amplifying sedation far beyond what either substance would cause alone.
Medical Uses Today
Chloral hydrate’s primary modern role is sedating young children for painless diagnostic procedures. It is used before brain imaging (MRI or CT scans), heart ultrasounds (echocardiograms), hearing tests, eye exams, and electroencephalograms (EEGs). These are all situations where a child needs to hold perfectly still but isn’t experiencing pain, so deep sedation without strong pain relief is sufficient.
It has also been used in pediatric dentistry to calm anxious children before procedures, though this application has drawn significant safety concerns. Several reported fatalities involved children given chloral hydrate before dental work, including cases where doses were administered at home before the child even arrived at the office, leaving no medical monitoring during the critical window when sedation deepens.
In adults, chloral hydrate was historically prescribed for insomnia and occasionally for managing alcohol withdrawal symptoms. The standard adult dose for sleep was 500 mg to 1 g taken 15 to 30 minutes before bedtime, with a firm ceiling of 2 g per day. For children, sedation doses ranged from 20 to 50 mg per kilogram of body weight depending on the procedure, with strict maximum limits. These narrow dosing windows hint at the drug’s core problem: the gap between a therapeutic dose and a dangerous one is uncomfortably small.
Side Effects and Risks
The most common side effects are stomach irritation and vomiting, which occur because the drug is caustic to the lining of the digestive tract. At higher doses or in overdose, it can cause erosive damage to the esophagus and stomach severe enough to cause bleeding or perforation.
The more serious risks involve the heart and lungs. Chloral hydrate can trigger dangerous heart rhythm disturbances, including rapid heartbeat, irregular beats, and potentially fatal rhythms like ventricular fibrillation. These cardiac effects have been reported even at therapeutic doses, carrying a small but real risk of sudden death. The drug also depresses breathing and relaxes the muscles that keep the airway open, creating a risk of respiratory arrest, particularly in children or when combined with other sedating substances.
In adults, the toxicity profile is roughly dose-dependent. Doses of 0.5 to 1 g produce normal sedation. At 1.5 to 2 g, sedation becomes excessive. The potentially lethal range starts at 3 g for adults, primarily because of cardiovascular collapse, though some individuals have survived ingestions as high as 35 g. In young children, potentially lethal heart rhythm disturbances have occurred with as little as 1.5 g. Liver and kidney damage can also occur in overdose, though less commonly.
Why It Has Fallen Out of Favor
No commercially manufactured oral formulation of chloral hydrate is widely available in the U.S. today. Pharmacies that still provide it typically compound it from raw powder or crystals into a liquid suspension, which introduces additional risks around dosing accuracy and quality control. The Institute for Safe Medication Practices lists it as a high-alert medication, meaning errors involving it carry an elevated risk of serious harm.
Several factors drove its decline. The narrow margin between an effective dose and a toxic one makes prescribing inherently risky, especially in children whose weight-based dosing must be precise. The cardiac risks are unpredictable, occurring even within recommended dose ranges. And the drug’s long duration of action means children remain sedated well after a procedure, requiring extended monitoring.
Newer alternatives, particularly benzodiazepines, offer wider safety margins and more predictable effects. However, head-to-head comparisons tell a more nuanced story. In studies comparing chloral hydrate to oral midazolam (a common benzodiazepine) for sedating infants before brain scans, chloral hydrate was significantly more effective at producing adequate sedation. Midazolam acted faster but frequently failed to sedate the child enough to complete the scan. For this reason, some pediatric guidelines still recommend chloral hydrate for specific painless imaging procedures where reliable sedation is essential and monitoring is available.
Legal Classification
Under the U.S. Controlled Substances Act, chloral hydrate is a Schedule IV substance, the same category as common benzodiazepines and certain barbiturates. This designation reflects a recognized medical use combined with a lower (but real) potential for dependence relative to Schedule II or III drugs. Prolonged use can lead to tolerance and physical dependence, and abrupt discontinuation after chronic use can trigger withdrawal symptoms similar to alcohol withdrawal.