The benzodiazepines, are a class of psychoactive drugs with varying hypnotic, sedative, anxiolytic, anticonvulsant, muscle relaxant and amnesic properties, which are mediated by slowing down the central nervous system. Benzodiazepines are potent anticonvulsants and have life-saving properties in the acute management of status epilepticus. The most commonly-used benzodiazepines for seizure control are lorazepam and diazepam. A meta-analysis of 11 clinical trials concluded that lorazepam was superior to diazepam in treating persistent seizures.] Although diazepam is much longer-acting than lorazepam, lorazepam has a more prolonged anticonvulsant effect. This is because diazepam is very lipid-soluble and highly protein-bound, and has a very large distribution of unbound drug, resulting in diazepam's having only a 20– to 30-minute duration of action against status epilepticus. Lorazepam, however, has a much smaller volume of distribution of unbound drug, which results in a more prolonged duration of action against status epilepticus. Lorazepam can therefore be considered superior to diazepam, at least in the initial stages of treatment of status epilepticus

Duration of action

Benzodiazepines are commonly divided into three groups by their half-lives: Short-acting compounds have a half-life of less than 12 hours, and have few residual effects if taken before bedtime, but rebound insomnia may occur and they might cause wake-time anxiety. Intermediate-acting compounds have a half-life of 12–24 hours, may have residual effects in the first half of the day. Rebound insomnia however is more common upon discontinuation of short-acting benzodiazepines. Daytime withdrawal symptoms are also a problem with prolonged usage of short-acting benzodiazepines, including daytime anxiety. Long-acting compounds have a half-life greater than 24 hours. Strong sedative effects typically persist throughout the next day if long-acting preparations are used for insomnia. Accumulation of the compounds in the body may occur. The elimination half-life may greatly vary between individuals, especially the elderly. Shorter-acting compounds are usually best for their hypnotic effects, whereas longer-acting compounds are usually better for their anxiolytic effects. Benzodiazepines with shorter half-lives tend to be able to produce tolerance and addiction quicker, as the drug does not last in the system for as long, with resultant interdose withdrawal phenomenon and next-dose craving. Although short-acting drugs are more commonly prescribed for insomnia, there are exceptions to the rules, such as alprazolam being prescribed as an anxiolytic more than a hypnotic, despite possessing a short half-life.

Mechanism of action

Benzodiazepines produce a range of effects from depressing to stimulating the central nervous system via modulating the GABAA receptor, the most prolific inhibitory receptor within the brain. The GABAA receptor is made up from 5 subunits out of a possible 19, and GABAA receptors made up of different combinations of subunits have different properties, different locations within the brain, and different activities relative to pharmacological and clinical effects.

Benzodiazepines bind at the interface of the α and γ subunits on the GABA A receptor. Benzodiazepine binding also requires that alpha subunits contain a histidine amino acid residue, (i.e., α1, α2, α3 and α5 containing GABAA receptors). For this reason, benzodiazepines show no affinity for α4 and α6 subunits containing GABAA receptors, which contain an arginine instead of a histidine residue. Other sites on the GABAA receptor also bind neurosteroids, barbiturates and certain anesthetics.

In order for GABAA receptors to be sensitive to the action of benzodiazepines, they need to contain both an α and a γ subunit, where the benzodiazepine binds at the interface. Once bound, the benzodiazepine locks the GABAA receptor into a conformation where the neurotransmitter GABA has much higher affinity for the GABAA receptor, increasing the frequency of opening of the associated chloride ion channel and hyperpolarizing the membrane. This potentiates the inhibitory effect of the available GABA, leading to sedatory and anxiolytic effects. As mentioned above, different benzodiazepines can have different affinities for GABAA receptors made up of different collection of subunits. For instance, benzodiazepines with high activity at the α1 are associated with sedation, whereas those with higher affinity for GABAA receptors containing α2 and/or α3 subunits have good anti-anxiety activity. Benzodiazepines also bind to glial cell membranes.  Benzodiazepines are full agonists at the benzodiazepine receptor producing anxiolytic and sedating properties. Compounds that bind to the benzodiazepine receptor and enhance the GABA receptor function are termed benzodiazepine receptor agonists and display sedative/hypnotic properties. Compounds that, in the absence of agonist, have no apparent activity but that competitively inhibit the binding of agonists to the receptor are called benzodiazepine receptor antagonists. Ligands that decrease GABA function are termed benzodiazepine receptor inverse agonists. Full inverse agonists have potent convulsant activities.

Some compounds lie somewhere between being full agonists or full antagonists, and are termed either partial agonists or partial antagonists. There has been interest in partial agonists for the benzodiazepine receptor with evidence that complete tolerance may not occur with chronic use, with partial agonists demonstrating continued anxiolytic properties with reduced sedation, dependence, and withdrawal problems.

The anticonvulsant properties of benzodiazepines may be in part or entirely due to binding to voltage-dependent sodium channels rather than benzodiazepine receptors. Sustained repetitive firing seems to be limited by benzodiazepines effect of slowing recovery of sodium channels from inactivation.