Depression (major depressive disorder) is a common and serious medical illness that negatively affects how you feel, the way you think and how you act. Fortunately, it is also treatable. Depression causes feelings of sadness and/or a loss of interest in activities you once enjoyed. It can lead to a variety of emotional and physical problems and can decrease your ability to function at work and at home.

Depression symptoms can vary from mild to severe and can include:

Feeling sad or having a depressed mood

Loss of interest or pleasure in activities once enjoyed

Changes in appetite — weight loss or gain unrelated to dieting

Trouble sleeping or sleeping too much

Loss of energy or increased fatigue

Increase in purposeless physical activity (e.g., inability to sit still, pacing, handwringing) or slowed movements or speech (these actions must be severe enough to be observable by others)

Feeling worthless or guilty

Difficulty thinking, concentrating or making decisions

Thoughts of death or suicide

Symptoms must last at least two weeks and must represent a change in your previous level of functioning for a diagnosis of depression.

Also, medical conditions (e.g., thyroid problems, a brain tumor or vitamin deficiency) can mimic symptoms of depression so it is important to rule out general medical causes.

Depression affects an estimated one in 15 adults (6.7%) in any given year. And one in six people (16.6%) will experience depression at some time in their life. Depression can occur at any time, but on average, first appears during the late teens to mid-20s. Women are more likely than men to experience depression. Some studies show that one-third of women will experience a major depressive episode in their lifetime. There is a high degree of heritability (approximately 40%) when first-degree relatives (parents/children/siblings) have depression.