Barbiturate
Definition
Cyclic ureide derivative of barbituric acid. Potentiates GABA-A. Sub-classed by duration of action: long (phenobarbital), intermediate (amobarbital), short (secobarbital, pentobarbital) and ultrashort (thiopental).
- Chemical class
- Cyclic ureide derivative of barbituric acid
- Mechanism
- Potentiates GABA-A receptor
- Long-acting example
- Phenobarbital
- Ultrashort-acting example
- Thiopental
Common questions
Why does duration of action matter for classifying barbiturates in a forensic toxicology context?+
Duration of action correlates with lipid solubility and redistribution rate in the body, which affects how quickly a drug produces effect and clears, so classifying a case barbiturate as long, intermediate, short, or ultrashort acting helps a toxicologist interpret the timing between ingestion, onset of impairment or death, and the concentration measured at autopsy.
How does barbiturate action at GABA-A differ from that of benzodiazepines, another common sedative class?+
Barbiturates directly increase the duration that the GABA-A chloride channel stays open and at high enough doses can open the channel even without GABA present, which produces a wider margin between therapeutic and lethal effect than benzodiazepines, whose action requires GABA to be present and is therefore generally considered safer in overdose.
Why are barbiturates less commonly encountered in current casework than in past decades?+
Their narrow therapeutic index and high overdose lethality relative to benzodiazepines led to a decline in clinical prescribing for anxiety and insomnia, though agents such as phenobarbital remain in use for seizure control and thiopental-class agents retain specialised anaesthetic or, in some jurisdictions, execution-related use, so they still appear in relevant toxicology casework.
Related terms
- Benzodiazepine
- Fused benzene plus seven-member diazepine ring. Allosteric modulator of GABA-A at the alpha-gamma interface. Reversed by flumazenil.
- FPN Reagent
- Ferric chloride, perchloric acid and nitric acid mixture. Red to orange-red colour with phenothiazines (chlorpromazine and relatives). Screen of choice for major...
- Schedule H1
- Forty-six substances notified by the 2013 amendment (Drugs and Cosmetics Fourth Amendment Rules, 2013, GSR 588(E) dated 30 August 2013, in force...
- Sedative-Hypnotic
- CNS depressant. Sedative dose calms (anxiolysis); hypnotic dose induces sleep; higher doses cause anaesthesia, respiratory depression and death.
- Tranquillizer
- Older umbrella term. Minor tranquillizers (benzodiazepines) treat anxiety. Major tranquillizers (phenothiazines, butyrophenones) are antipsychotics.
- Z-Drug
- Non-benzodiazepine hypnotic (zolpidem, zopiclone, eszopiclone) that binds the alpha-1 subunit of GABA-A selectively. Same date-rape and stupefying-robbery potential as the BZDs.
- Zwikker / Dille-Koppanyi Reagent
- Cobalt salt (nitrate or acetate) plus isopropylamine. Violet to blue-violet colour with barbiturates. Workhorse barbiturate screen since the 1940s.