Major depression is the most serious kind of depression. It is characterized by a constant sad mood and loss of interest in all activities that lasts for at least two weeks.

Other features include a profound lack of energy, a distorted, negative self-image, and deep sadness or hopelessness. It can be moderate to severe and pass in a matter of months or last for years without treatment. Major depression is thought to be caused by a disturbance in brain chemistry rather than a psychological reaction to external events.