Barbiturates

Abstract

The barbiturates were used historically as hypnotic agents but except for a few specialist uses they have been superseded in clinical practice by the benzodiazepines. With continued use, tolerance develops to the sedative and hypnotic effects but not the lethal respiratory depressant effects, and thus the therapeutic index decreases with time. Barbiturates are metabolized by and induce microsomal enzymes in the liver, thereby decreasing their own activity and that of many other drugs. Most are excreted as metabolites in urine with a smaller amount of unchanged drug. Half-lives vary, ranging from 8-42 hours for amylobarbitone to 50-150 hours for phenobarbitone. Barbiturate use causes dependence and they are drugs of abuse. Their abuse has declined in line with clinical use and now occurs sporadically. The US Department of Defense cut-off concentration for radioimmunoassay of barbiturates in urine is 0.2 μg/ml. For confirmation of any barbiturate except phenobarbitone by GC-MS the value is 0.2 μg/ml.