In major depression, sometimes referred to as unipolar or clinical depression, people have some or all of the symptoms listed below for at least 2 weeks but frequently for several months or longer. Episodes of the illness can occur once, twice, or several times in a lifetime.

In dysthymia, the same symptoms are present though milder and last at least 2 years. People with dysthymia are frequently lacking in zest and enthusiasm for life, living a joyless and fatigued existence that seems almost a natural outgrowth of their personalities. They also can experience major depressive episodes.