What Is Major Depressive Disorder?

Written by the Able Psychiatry clinical team

Updated September 27, 2026

Major depressive disorder is a mental health condition diagnosed when a person has a depressed mood or a loss of interest or pleasure in most activities, present most of the day, nearly every day, for at least two weeks, along with other symptoms severe enough to interfere with daily life. It is also called major depression or clinical depression.

Key takeaways

• A diagnosis requires a depressed mood or loss of interest most of the day, nearly every day, for at least two weeks, per NIMH’s diagnostic guidance.
• NIMH’s analysis of 2021 national survey data found an estimated 21.0 million U.S. adults, or 8.3% of all adults, had at least one major depressive episode in the past year.
• It is a distinct diagnosis from persistent depressive disorder (dysthymia), which involves milder symptoms lasting two years or longer.
• Standard treatments include psychotherapy, antidepressant medication, and, when those aren’t enough, brain stimulation therapies such as TMS.

How major depressive disorder is diagnosed

To be diagnosed, a person must have symptoms most of the day, nearly every day, for at least two weeks, and one of those symptoms must be a depressed mood or a loss of interest or pleasure in most activities. Children and adolescents may show irritability rather than sadness. NIMH lists the common signs as including a persistent sad or empty mood, loss of interest in hobbies and activities, fatigue, difficulty concentrating, changes in sleep or appetite, feelings of guilt or worthlessness, and thoughts of death or suicide.

If you are having thoughts of suicide or harming yourself, call or text 988 (Suicide & Crisis Lifeline) now, or call 911 or go to the nearest emergency room.

A clinician also has to rule out other explanations. Certain medications and medical conditions, such as thyroid disorders, can cause the same symptoms, so a diagnostic evaluation typically includes a physical exam, an interview, and lab work before the symptoms are attributed to major depressive disorder.

Types and presentations of major depressive disorder

• With severe impairment: NIMH’s 2021 national survey data found about 5.7% of U.S. adults had a major depressive episode severe enough to cause serious disruption to work, home life, or relationships, compared with 8.3% who met criteria overall.
• Single vs. recurrent episode: some people have one depressive episode in their lifetime; others have repeated episodes over time.
• Treatment-resistant: NIMH defines treatment-resistant depression as depression that doesn’t improve after trying at least two antidepressants — see treatment-resistant depression: next steps for what comes after that point, including augmentation therapy.
• With a seasonal pattern, psychotic features, or perinatal onset: NIMH describes seasonal affective disorder, depression with psychosis, and perinatal depression as related presentations that share the same core mood and interest symptoms but differ in timing or severity.

Example of major depressive disorder

The real-world scale and severity of major depressive disorder is documented in NIMH’s analysis of the 2021 National Survey on Drug Use and Health. Researchers scored impairment using the Sheehan Disability Scale, which rates how much a condition disrupts four areas of daily functioning — home management, work, close relationships, and social life — on a 0-to-10 scale. A rating of 7 or higher in even one of those domains counted as severe impairment. Applying that standard, an estimated 14.5 million U.S. adults, or 5.7% of all adults, had a major depressive episode with severe impairment in the past year, and 74.8% of that severely impaired group received treatment.

What people get wrong

Patients often assume major depressive disorder has to look like visible sadness or crying. In practice it can show up as irritability, physical complaints like headaches or stomach trouble, or simply feeling flat and checked out of things that used to matter. Loss of interest or pleasure — anhedonia — carries just as much diagnostic weight as low mood, and for some patients it’s the more noticeable symptom. Ruling out a medical cause, like a thyroid problem, is a routine part of a careful evaluation, not an afterthought.

Major depressive disorder vs. persistent depressive disorder

	Major depressive disorder	Persistent depressive disorder
Duration	At least 2 weeks per episode; can be a single episode or recurring	At least 2 years in adults, ongoing
Symptom severity	More numerous, more acute symptoms	Milder symptoms that persist longer
Also called	Major depression, clinical depression	Dysthymia, dysthymic disorder

Both conditions share depressed mood or loss of interest as a core feature, per NIMH. The difference is mainly one of duration and intensity rather than a difference in kind, and some people experience both — a chronic low-grade depression punctuated by a more severe episode.

Frequently asked questions

Is major depressive disorder the same thing as feeling sad?
No. Everyone feels sad sometimes, and those feelings usually pass. Major depressive disorder involves a depressed mood or loss of interest that persists most of the day, nearly every day, for at least two weeks, and that interferes with daily activities like sleeping, eating, or working.

How long do symptoms need to last for a diagnosis?
At least two weeks, most of the day and nearly every day, with one of the required symptoms being a depressed mood or a loss of interest or pleasure in most activities, according to NIMH.

How common is major depressive disorder in the U.S.?
NIMH’s analysis of 2021 survey data estimated that 21.0 million U.S. adults, or 8.3% of all adults, had at least one major depressive episode in the past year, with higher rates among females and younger adults.

What’s the difference between major depressive disorder and persistent depressive disorder?
Major depressive disorder is diagnosed by episode: at least two weeks of more severe symptoms. Persistent depressive disorder (dysthymia) involves milder symptoms that last two years or longer, per NIMH.

The bottom line

Major depressive disorder is defined by duration and pattern, not just by how sad someone looks: a depressed mood or loss of interest most of the day, nearly every day, for at least two weeks, plus other symptoms that get in the way of daily life. It’s common, treatable, and distinct from milder, longer-lasting conditions like persistent depressive disorder — an accurate diagnosis is what points to the right next step.

Sources

• Depression, National Institute of Mental Health (NIH Publication No. 24-MH-8079, revised 2024)
• Major Depression, National Institute of Mental Health (statistics last updated July 2023, based on 2021 National Survey on Drug Use and Health)
• Depression, National Institute of Mental Health (last reviewed December 2024)

Related terms

• Anhedonia
• Augmentation therapy
• Medication management
• Psychiatric evaluation
• Telepsychiatry
• Treatment-resistant depression
• How psychiatrists diagnose conditions

Living with major depressive disorder? Learn about TMS treatment at Able Psychiatry in Chicago, or schedule an appointment to talk through your options.