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What Is Depression?

By William Marchand, M.D. ; Updated October 19, 2017

If you have been feeling sad for two weeks or more you might be suffering from depression. Depression is a psychiatric illness that causes sadness and difficulty experiencing pleasure. It can also impact sleep, appetite and energy levels as well as impair thinking. Most concerning is the fact that some people suffering from depression develop suicidal thoughts.

Getting Help

If you (or someone you know) are having thoughts of suicide or self-harm, it’s an emergency that requires immediate evaluation by a medical or mental-health professional. Several options exist for getting help immediately. One of these is the National Suicide Prevention Lifeline ( This is a free, 24-hour hotline available to people experiencing emotional crises or having thoughts of harming themselves or others. The number is 1-800-273-TALK (8255). Other options include calling 911 or having a family member or friend take you to the nearest emergency room. You can also call your mental-health or medical provider (if you have one). But if a provider is not immediately available to help, use one of the above methods.

Depressive Disorders

There are several depressive disorders as outlined in the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM 5), published by the American Psychiatric Association. These include major depressive disorder, persistent depressive disorder, premenstrual dysphoric disorder, disruptive mood dysregulation disorder, depressive disorder due to a medical condition and substance or medication-induced depressive disorder.

Major depressive disorder is an illness with moderate to severe symptoms that persists for at least two weeks, but typically much longer. Persistent depressive disorder is associated with milder symptoms that persist for at least two years. Premenstrual dysphoric disorder is a condition in which mood symptoms occur in the week before the onset of your period. Disruptive mood dysregulation disorder manifests as irritable mood with temper tantrums and first occurs in childhood. Depressive disorders can also be caused by medications, substance use or other medical conditions. In addition, depression can be associated with the season (seasonal affective disorder), psychosis (delusions and/or hallucinations) and anxiety.

The diagnosis of a specific depressive disorder can be made by a medical or mental-health professional. However, knowing the symptoms of depression is important in order to know when a professional evaluation is indicated. These are discussed in the Signs and Symptoms of Depression section.

If you have or think you might have depression, it is critical to seek help and start treatment as quickly as possible.

How Common is Depression?

It is important to know about depression because of its prevalence. According to the World Health Organization, an estimated 350 million people are affected worldwide. In regard to major depressive disorder, a report by the U.S. Substance Abuse and Mental Health Services Administration indicated that 1 in 12 adults in the United States experienced a major depressive episode during the prior year. Overall, it is thought that 14 million to 16 million people experience a major depressive episode each year in the U.S. For reasons that we don’t understand, depression is about twice as common among women as it is among men. The prevalence range of major depression is about 3 to 5 percent for men and 6 to 12 percent for women. So about one in every 10 women experience depression sometime in their life as compared to about one in 20 men. For persistent depressive disorder, the prevalence is 3 to 4 percent, thus about one in 25 individuals suffer from this condition.

Depressive disorders are very common conditions. Almost everyone in the U.S. has either experienced depression or knows a friend or family member who has. This condition can cause severe impairment or death. The good news is that effective treatments are available, and recovery is likely with appropriate treatment.

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