the first synthetically produced sedative-hypnotic drug, commonly used in the late 19th century to treat insomnia and still occasionally used to reduce anxiety or produce sleep before surgery. Chloral hydrate acts as a depressant on the central nervous system (nervous system, human), with sedative effects similar to those of barbiturates (barbiturate).

Chloral hydrate (trichloroacetaldehyde monohydrate) was first synthesized in 1832, but it was not introduced into medicine until 1869, when Mathias E.O. Liebreich discovered its effectiveness in inducing sleep. A therapeutic dose produces a deep sleep lasting four to eight hours with few aftereffects, but habitual use of the drug results in addiction—a fact quickly noted in the medical literature of the late 19th and the early 20th century. Symptoms of overdose may include deep stupor, dilation of blood vessels, fall in blood pressure and body temperature, and slowed respiration. In a severe overdose, death usually occurs within 5 to 10 hours. Chloral hydrate was the primary ingredient, along with alcohol, of the “knockout drops” or “Mickey Finns” of popular lore. With the development of safer and more effective drugs, the use of chloral hydrate has declined. When used, it is administered as gel capsules or rectal suppositories.