Barbiturates
Old sedative-hypnotics that act on GABA-A receptors. Now used mainly for seizures and anaesthesia, because overdose and dependence are dangerous.
What they are
Derivatives of barbituric acid, once the main sleeping pills and sedatives, now largely replaced by benzodiazepines. They are grouped by duration: long-acting phenobarbital (and primidone, which is partly converted to it) for epilepsy; short-acting pentobarbital for refractory seizures, induced coma and sedation; ultra-short-acting thiopental and methohexital for anaesthesia. Phenobarbital is on its antiseizure-medicines page and thiopental and methohexital on the intravenous-anaesthetics page.
Mechanism of action
Barbiturates bind a site on the GABA-A receptor distinct from the benzodiazepine site. They prolong the time the chloride channel stays open when GABA binds, and at higher concentrations open it directly; they also reduce glutamate (AMPA) excitation. CNS depression therefore rises with dose without a ceiling, from sedation to anaesthesia, coma, respiratory arrest and death. They also induce liver CYP enzymes and ALA synthase.
Target: GABA-A receptor (barbiturate site)
Drugs in this class
Others in the class
- Phenobarbital (antiseizure medicines)
- Primidone (antiseizure medicines)
- Thiopental (intravenous anaesthetics)
- Methohexital (intravenous anaesthetics)
- Secobarbital, amobarbital, butalbital
Test yourself
Think of the answer, then tap the question to see it.
What is the key difference between barbiturates and benzodiazepines at the GABA-A receptor?
Barbiturates prolong the duration of channel opening and can open it directly; benzodiazepines increase opening frequency only when GABA is present.
Why are barbiturates contraindicated in acute intermittent porphyria?
They induce ALA synthase, increasing porphyrin precursors and triggering attacks.
Which barbiturate is used for epilepsy, and which for anaesthesia induction?
Phenobarbital (epilepsy); thiopental or methohexital (anaesthesia).
Why can a patient on phenobarbital have an unplanned pregnancy while taking the pill?
Enzyme induction speeds the breakdown of contraceptive hormones.
Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.
Sources: PENTOBARBITAL SODIUM injection, solution (DailyMed (U.S. National Library of Medicine), 2026); ATCDDD - ATC/DDD Index (WHO Collaborating Centre for Drug Statistics Methodology, 2026); Human GABAA receptor alpha1-beta2-gamma2 subtype in complex with GABA plus phenobarbital (RCSB Protein Data Bank, 2020).
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Representative model, for educational purposes only.