Major Depressive Disorder (MDD)

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Major Depressive Disorder (MDD)

THC Editorial Team December 29, 2020
Charles Warren Eaton, Woods in Winter, 1886, Courtesy National Gallery of Art, Washington (article on major depressive disorder)
Charles Warren Eaton, Woods in Winter, 1886, Courtesy National Gallery of Art, Washington


What Is Major Depressive Disorder?

According to the American Psychiatric Association, major depressive disorder (MDD) is a serious illness that is common and treatable. Major depression negatively impacts how someone thinks, feels, and acts and is characterized by feelings of sadness or loss of interest in activities that they once enjoyed.1,2

Depression affects hundreds of millions of people around the world. It occurs along a continuum of severity that ranges from relatively mild episodic states of depressed mood to serious, long-term depressive symptoms that can greatly impact the quality of a person’s life. When someone experiences chronic, severe symptoms that require professional treatment, their disorder will typically be referred to as major depressive disorder or clinical depression.

Major depressive disorder is a prevalent condition. According to the National Institute of Mental Health, approximately 17.3 million adults in the United States experienced an episode of major depressive disorder in 2017 alone. Furthermore, 13.1% of people between the ages of 18 and 25 experienced an episode of major depressive disorder, a higher percentage than in any other age group. Women were likelier than men to have experienced an episode (8.7% versus 5.3%).1

Major depressive disorder can affect people of all ages. Also known as unipolar depression, it is one of several types of depression defined in the Diagnostic and Statistical Manual of Mental Disorders (5th edition; DSM-5). Another disorder defined in the DSM-5 is bipolar disorder, in which one criterion is the occurrence of a major depressive episode.

What Are the Symptoms of Major Depressive Disorder?

According to Harvard Medical School, the symptoms of major depressive disorder are much more severe than simply feeling sad or having a bad day; they are chronic and serious, and they last for 2 weeks or longer.3 In many cases, people who suffer from major depressive disorder experience symptoms for years. Although symptoms vary from person to person, most people experience some combination of the following:

  • mood that is distinctly depressed or irritable
  • loss of the ability to experience pleasure or loss of interest
  • decreases or increases in weight or appetite
  • increases or decreases in sleep
  • noticeable agitation or slowness
  • fatigue, tiredness, and loss of energy
  • feelings of worthlessness or guilt
  • inability to concentrate or difficulty making decisions
  • suicidal thoughts, persistent thoughts of death, or suicidal plans and attempts

Causes and Risk Factors of Major Depressive Disorder

Several risk factors for major depressive disorder have been identified. The National Institute of Mental Health reports that research indicates depression may be caused by environmental, genetic, biological, and psychological factors.4

For example, research published in the Canadian Medical Association Journal looked at the neurobiological basis of major depression and the role of monoamines—types of neurotransmitters including norepinephrine and serotonin—in the etiology of the disease. Although many people have low levels of norepinephrine and serotonin in their systems, the researchers found that lower levels of these monoamines do not result in depression in all people. The researchers believe that genetic variations responsible for how monoamines are metabolized, combined with other factors, might play a role in the development of depression.5 Other research published in the Journal of the American Medical Association found that a biochemical imbalance of neurotransmitters in the brain may be a risk factor for depression.6

Additional identified risk factors for the development of major depressive disorder include having a chronic illness, having a personal or family history of depression, or undergoing major life changes, stress, or trauma.

How Is Major Depressive Disorder Diagnosed?

Major depressive disorder is normally diagnosed by a mental health professional or a medical doctor through a clinical interview. In this process, the doctor or mental health professional may ask about the patient’s symptoms and medical history and then use the information to check the diagnostic criteria as outlined in the DSM-5.

The Substance Abuse and Mental Health Services Administration reports that medical professionals may diagnose major depressive disorder according to the DSM-5 if a patient has experienced associated symptoms every day for 2 weeks or longer and meets five or more of the listed criteria—which must include either a persistent, depressed mood or a loss of interest in previously enjoyed activities.7 Although the symptoms previously listed can be indicators of major depressive disorder, the individual must meet specific criteria for major depressive disorder to be officially diagnosed. These criteria include the following:

  • an unintentional and significant weight loss or weight gain of at least 5% in the past month or a loss of or increase in appetite
  • disturbance of sleep, including insomnia or hypersomnia
  • severe, observable psychomotor changes
  • fatigue, loss of energy, tiredness, or reduced efficiency during routine tasks
  • feelings of worthlessness or excessive and inappropriate guilt
  • difficulty concentrating, thinking, or making decisions
  • suicidal ideations, persistent thoughts of death, or suicide attempts
  • significant impairment or distress in work-related, social, or other important areas of functioning as a result of the symptoms
  • lack of a previous manic or hypomanic episode

A medical doctor should evaluate a person who is suspected of having major depressive disorder to ensure that the symptoms are not the result of a different psychiatric condition (e.g., a psychotic disorder or schizophrenia), a medical condition, or a medication.

Treatments for Major Depressive Disorder

Major depressive disorder is common; many people experience at least one episode of depression during their lives. Fortunately, depression responds well to treatment, which allows patients to move forward and to enjoy happier, healthier lives.2

Harvard Medical School reports that the most effective treatment approach for major depressive disorder is a combination of psychotherapy and medication. Selective serotonin reuptake inhibitors, or SSRIs, are often prescribed and take 2–6 weeks for patients to begin experiencing improvements. Several psychotherapeutic approaches may be used, including cognitive behavioral therapy or psychodynamic therapy. In severe cases in which people do not respond to other treatment, electroconvulsive therapy may be used.3


  1. American Psychiatric Association. (2020). What is depression?
  2. National Institute of Mental Health. (2019). Major depression. U.S. Department of Health and Human Services, National Institutes of Health.,-Figure%201%20shows&text=An%20estimated%2017.3%20million%20adults,compared%20to%20males%20(5.3%25)
  3. Harvard Health Publishing, Harvard Medical School. (2018, December). Major depression.
  4. National Institute of Mental Health. (2018). Depression. U.S. Department of Health and Human Services, National Institutes of Health.
  5. aan het Rot, M., Mathew, S. J., & Charney, D. S. (2009). Neurobiological mechanisms in major depressive disorder. Canadian Medical Association Journal, 180(3), 305–313.
  6. Weissman, M. M., Berry, O. O., Warner, V., Gameroff, M. J., Skipper, J., Talati, A., Pilowsky, D. J., &Wickramaratne, P. (2016). A 30-year study of 3 generations at high risk and low risk for depression. JAMA Psychiatry, 73(9), 970–977.
  7. Substance Abuse and Mental Health Services Administration. (2016). DSM-5 changes: Implications for child serious emotional disturbance.

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