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Depression is one of the most common mood disorders. Everyone experiences depressed moods from time to time. More commonly referred to as "having the blues" or "being down in the dumps," the sad or depressed mood usually lasts for only a short period. When the feeling persists for weeks without apparent reason, however, it may be a sign of major depression, a psychiatric disorder.

The symptoms of major depression include a sad or depressed mood or a marked lack of interest and pleasure in almost all activities. This feeling persists for most of the day, nearly every day, for at least two weeks. In addition, many or all of the following symptoms occur: (1) loss of appetite; (2) fatigue (tiredness); (3) difficulty...