Antidepressants are a diverse group of drugs that demonstrate a capacity to produce improvement in the symptoms of clinical depression, and they are used to treat the abnormal mood states that characterize depressive illnesses. The word depression is used commonly to describe a state of sadness; but health professionals use the term in a more restricted or defined manner to describe several psychiatric disorders characterized by abnormal moods. One of these is bipolar disorder, in which periods of depression (marked by dejection, lack of energy, inactivity, and sadness) alternate with periods of manic behavior (marked by abnormally high energy levels and increased activity). Another is major depression, which is often a recurring problem characterized by severe and prolonged periods of depression without the manic swing. A third is dysthymia, a chronic mood state characterized by depression and irritability, which was once referred to as depressive neurosis. The signs...