CNS DEPRESSANTS depress the CNS. In practice, very diverse agents can be grouped under this heading and it does not describe any specific type of drug action. Most of the agents that depress neuronal activity in the brain or spinal cord are dealt with under specific headings. The properties of some of these classes will be summarized here.

GENERAL ANAESTHETICS are either inhaled or injected agents and produce insensibility, mostly to alleviate pain during surgical procedures (e.g. halothane, thiopentone sodium).

HYPNOTICS induce sleep and a wide range of chemical types may be used. The older agents, such as the barbiturates, were often SEDATIVE agents used at a higher dose, but they readily produced dangerous respiratory depression and are now much less commonly used (e.g. amylobarbitone, chloral hydrate, chlormethiazole, triclofos). Increasingly, the drugs of choice are anxiolytic/minor tranquillizers at a somewhat higher dosage (vide infra).

Tranquillizers depress the CNS. The need for the term came with the introduction of drugs having rather more subtle effects on mood and behaviour than the barbiturates. However, it soon became necessary to divide the category into minor tranquillizers and major tranquillizers. Currently, it is thought simpler to describe the drugs largely according to usage, so the term minor tranquillizers is used more or less synonymously with ANXIOLYTIC AGENTS. The major tranquillizers have had other words coined to describe their sort of activity (e.g. neuroleptic, thymoleptic’) but are largely used as antipsychotics, though some members have actions making them valuable for tranquillizing severely agitated patients (e.g. chlorpromazine).

ANTICONVULSANTS are normally used as antiepileptic agents, which produce very little generalized depression of the brain and instead target only hyperexcitable groups of neurons that are producing inappropriate bursts of firing. However, some may produce drowsiness. Examples include carbamazepine, ethosuximide, phenobarbitone and valproic acid. Some of the agents used as anticonvulsants to treat stimulant drug or chemical poisoning are, at the doses used, rather more depressant, e.g. diazepam.

Narcotic analgesics may be depressant at higher doses. Indeed, morphine can be very sedative and may induce sleep. However, with some it is feasible to produce good analgesia without generalized depression of the CNS. See OPIOID ANALGESICS.

Many other therapeutic or non-therapeutic agents may produce CNS depression at some part of their dosage range. For instance, many antihistamines (HISTAMINE H1-RECEPTOR ANTAGONISTS), when used as antiallergic agents or in the treatment of motion sickness, may cause drowsiness or sleep, which can be a considerable disadvantage. In overdose, many agents may cause dangerous depression, particularly in combination with socially-used depressants such as ethanol.