Major depressive disorder, also known as clinical depression or MDD, is a form of depression marked by extreme periods of sadness. About 7 percent of American adults experience an episode of MDD each year.

Many of the symptoms of MDD are mental, including feeling restless and worthless. It can also take a physical toll on your body, leaving you tired and affecting your appetite and weight. While the cause of MDD is unknown, it can be treated through medications, lifestyle changes, and therapy.