benzodiazepine
A benzodiazepine is a sedative drug that works like a dimmer switch on the brain's main 'brake.' It doesn't open the brake by itself, but when the body's own inhibitory signal is present, the benzodiazepine makes that brake press down harder and more often, so anxious or overactive brain circuits quiet down.
Mechanistically, benzodiazepines are positive allosteric modulators of the GABA-A receptor, a chloride ion channel. By binding to a site separate from where GABA binds, they increase the frequency of channel opening when GABA is present, letting more chloride into the neuron and making it less excitable. Because they need GABA to act, their effect has a ceiling. Familiar examples include diazepam, lorazepam, alprazolam, and midazolam.
Benzodiazepines are used for anxiety, panic, insomnia, alcohol withdrawal, seizures and status epilepticus, and as premedication before procedures. They differ mainly in onset and duration, which guides the choice for a given use.
Their main drawbacks are sedation, impaired memory and coordination, and — with regular use over weeks — tolerance, physical dependence, and a withdrawal syndrome on stopping. The reversal antidote flumazenil can displace them from the receptor, but it must be used cautiously because it can precipitate seizures in dependent patients.
In the emergency department, intravenous lorazepam is given to stop a prolonged seizure (status epilepticus) by rapidly boosting GABA-mediated inhibition.
Fast-acting benzodiazepines are first-line for terminating acute seizures.
The ceiling on their GABA enhancement is why benzodiazepines are far safer in overdose than barbiturates when taken alone — but this margin disappears when they are mixed with other CNS depressants.