An antidepressant is a medication used primarily in the treatment of clinical depression. Modern antidepressants are not stimulants and are not generally addictive nor do they produce tolerance although sudden withdrawal may produce adverse effects. Conversely, stimulants are not usually considered as first line treatment for depression. Antidepressants create little if any immediate change in mood and require between several days and several weeks to take effect.

The therapeutic effects are believed to be related to an effect on neurotransmitters, particularly serotonin and norepinephrine. Selective serotonin reuptake inhibitors specifically prevent the reuptake of serotonin (thereby increasing the level of serotonin at synapses in the brain), whereas earlier monoamine oxidase inhibitors block the destruction of neurotransmitters by enzymes which break them down. Recent research published by the National Institutes of Health in the USA suggests that antidepressants may also have some effect by promoting neurogenesis in the hippocampus.

Note that antidepressants may actually make the mania component of bipolar disorder worse, and should be used with great care in the treatment of that disorder, usually in conjunction with mood stabilisers. Their use should be monitored by a psychiatrist, but in countries such as Britain, New Zealand and the United States, primary care physicians are able to prescribe antidepressents without consulting a psychiatrist. In particular, it has been noted that the most dangerous period for suicide in a patient with depression is immediately after treatment has commenced, as antipressants may reduce the symptoms of depression such as psychomotor retardation or lack of motivation before mood starts to improve.

Antidepressants can often cause side effects, and an inability to tolerate these is the most common cause of discontinuing the medication. Sexual dysfunction is a very common side effect. More recent drugs have fewer side effects, but many patients are reporting severe side effects when they try to stop taking some antidepressants, particularly paroxetine. Additionally, certain patients do not respond to antidepressant drugs.

Like many psychiatric drugs, antidepressants were discovered by accident. The first antidepressants, imipramine a tricyclic and isoniazid a monoamine oxidase inhibitor (MAOI), were found in the 1950s and the latter was originally intended for the treatment of tuberculosis. These drugs were found to have the side effect of improving the patients' mood. However, the newer SSRI antidepressants were early examples of rational drug design.

Many antidepressants also are used for the treatment of anxiety disorders, and tricyclic antidepressants are used in the treatment of chronic pain disorders such as Myofacial Pain Syndrome and post-herpetic neuralgia.

Classes of antidepressant:

See also: List of antidepressants

Also despite controversy, alternative treatments for depression such as the herbal remedy St John's wort have gained popularity in recent years, although its effectiveness is open to debate.

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