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. It 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.

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

Depression Is Different From Sadness or Grief/Bereavement

The death of a loved one, loss of a job or the ending of a relationship are difficult experiences for a person to endure. It is normal for feelings of sadness or grief to develop in response to such situations. Those experiencing loss often might describe themselves as being “depressed.”

But being sad is not the same as having depression. The grieving process is natural and unique to each individual and shares some of the same features of depresion. Both grief and depresion may involve intense sadness and withdrawal from usual activities. They are also different in important ways:

• In grief, painful feelings come in waves, often intermixed with positive memories of the deceased. In major depression, mood and/or interest (pleasure) are decreased for most of two weeks.
• In grief, self-esteem is usually maintained. In major depresion, feelings of worthlessness and self-loathing are common.
• In grief, thoughts of death may surface when thinking of or fantasizing about “joining” the deceased loved one. In major depression, thoughts are focused on ending one’s life due to feeling worthless or undeserving of living or being unable to cope with the pain of depression.

Grief and depression can co-exist For some people, the death of a loved one, losing a job or being a victim of a physical assault or a major disaster can lead to depression. When grief and depression co-occur, the grief is more severe and lasts longer than grief without depression.  Distinguishing between grief and depression is important and can assist people in getting the help.

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Sources:
https://www.psychiatry.org/patients-families/deprssion/what-is-depresion