Nervous system stimulants and depressants act predominantly on the central nervous system - the brain and major nerves leaving and entering the brain. The easiest way to think of these drugs (although this is an oversimplification of a complex and not fully understood process) is that they are chemicals that increase or decrease the activity of the nerves by altering the levels of chemicals known as neurotransmitters, which carry messages between the nerves.

Stimulants

Stimulant drugs increase alertness and heighten awareness. They work in an area of the brain called the reticular activating system (RAS), which lies in the brain stem. The RAS controls wakefulness by sending signals to the upper part of the brain (the cerebral cortex), which controls thought processes.

Activity in the RAS is controlled by the balance of inhibitory chemicals, such as gamma aminobutyric acid (GABA), and excitatory chemicals, including noradrenaline. Stimulant drugs trigger the release of noradrenaline by the brain cells, increasing RAS activity and so increasing the 'thinking' activity of the cerebral cortex.

The best-known stimulant drug is caffeine. We often drink coffee to stay awake or keep our brains going when we're tired.

Other stimulants include amphetamines and similar drugs once used in slimming to control appetite, but now banned because of their side-effects. Stimulants are also used for narcolepsy (a condition of excessive sleepiness) and hyperactivity (it sounds paradoxical, but they can help to increase the attention span).

The risks of stimulants are considerable and many are classified as controlled drugs. They're addictive and in the long term the brain becomes tolerant to them, so increasing doses are needed to have the same affect. They can cause palpitations, anxiety, hallucinations and seizures. During use, natural stimulant production slows, so if the drug is stopped there may be depression and lethargy.

Depressants

Nervous system depressants tend to have the opposite effect. They include barbiturates (rarely used now as sleeping drugs, but still sometimes used in anaesthesia), benzodiazepine drugs such as valium, and some newer sleeping drugs of a non-barbiturate, non-benzodiazepine type.

They're used mostly to induce sleep or treat anxiety and related symptoms. They work in the brain by reducing communication between the nerve cells and so decreasing brain activity and inducing sleepiness.

Different drugs work in different ways and on different areas and chemicals in the brain. Benzodiazepines work on the RAS, by sticking to the same receptors on the nerves that GABA works at, increasing the inhibitory balance.

Again, there are side-effects and risks, especially of physical and psychological dependence. Some types, such as the barbiturates, may be especially dangerous in overdose.

Depressant drugs should only be used in short courses, to help anxiety and sleep problems during periods of intense stress, such as bereavement.

This article was last medically reviewed by Dr Trisha Macnair in January 2008