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# Barbiturates

Old sedative-hypnotics that act on GABA-A receptors. Now used mainly for seizures and anaesthesia, because overdose and dependence are dangerous.

Updated Nov 12, 2026

## 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)

## Key points

- Barbiturates prolong the DURATION of chloride-channel opening; benzodiazepines increase its FREQUENCY.
- No ceiling effect: overdose causes coma, hypotension, hypothermia and respiratory arrest; care is supportive (airway, ventilation), with urine alkalinisation or haemodialysis in severe cases.
- Strong enzyme inducers: they lower the effect of warfarin, steroids and hormonal contraceptives.
- Contraindicated in acute porphyria (they induce ALA synthase).
- Dependence: sudden withdrawal after long use can cause delirium, seizures and death.

## Drugs in this class

- [**Pentobarbital** A short-acting barbiturate given by injection for refractory seizures and sedation. Strongly alkaline, habit-forming… GABA-A receptor (barbiturate site)](https://durgical.com/en/pharmacology/pentobarbital/)

## 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.

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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](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=d375b92a-baa1-4d63-9ad1-1467dcfde7d5) (DailyMed (U.S. National Library of Medicine), 2026); [ATCDDD - ATC/DDD Index](https://atcddd.fhi.no/atc_ddd_index/?code=N05CA) (WHO Collaborating Centre for Drug Statistics Methodology, 2026); [Human GABAA receptor alpha1-beta2-gamma2 subtype in complex with GABA plus phenobarbital](https://www.rcsb.org/structure/6X3W) (RCSB Protein Data Bank, 2020).

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Representative model, for educational purposes only.

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