MDD stands for major depressive disorder, a mental health condition that goes well beyond ordinary sadness. It affects roughly 5.7% of adults worldwide and is defined by persistent symptoms lasting at least two weeks that interfere with daily life. Unlike temporary low moods that everyone experiences, MDD changes how you think, sleep, eat, and function in ways that don’t resolve on their own.
How MDD Is Diagnosed
A diagnosis requires at least five specific symptoms present during the same two-week period. At least one of those symptoms must be either a persistently depressed mood or a loss of interest or pleasure in activities you used to enjoy. The remaining symptoms include changes in appetite or weight, sleep problems (too much or too little), physical restlessness or feeling slowed down, fatigue or loss of energy, difficulty thinking or concentrating, feelings of worthlessness or excessive guilt, and thoughts of death or suicide.
These symptoms also need to cause real disruption to your social life, work, or other important areas of functioning. A bad week after a stressful event doesn’t meet the threshold. The pattern has to be consistent and pervasive enough that it changes how you operate day to day.
What MDD Feels Like
The experience varies widely from person to person, which is part of what makes it tricky to recognize. Some people feel a heavy, persistent sadness. Others describe it more as emotional numbness, where nothing feels enjoyable or meaningful anymore. That second experience, called anhedonia, is one of the hallmark features. Hobbies, relationships, food, sex: things that once brought satisfaction simply stop registering.
Physical symptoms are common and often overlooked. Fatigue that sleep doesn’t fix, aches without a clear cause, difficulty concentrating enough to read a paragraph or follow a conversation. Some people sleep 12 hours and still feel exhausted. Others can’t fall asleep at all. Appetite swings in both directions, with some people losing weight rapidly and others eating compulsively. The cognitive effects, sometimes called “brain fog,” can make people worry they’re developing dementia when the real issue is depression.
What Causes It
MDD doesn’t have a single cause. Genetics account for an estimated 30% to 50% of the risk, based on twin studies that put the heritability at around 37%. Interestingly, shared family environment (growing up in the same household) contributes relatively little, somewhere between 0% and 11% of the variance. This means that while the condition runs in families, the mechanism is more genetic than environmental in the traditional sense.
Brain chemistry plays a central role. Disruptions in serotonin, a chemical messenger that helps regulate mood, sleep, and appetite, are thought to be especially important. But serotonin is only part of the picture. Stressful life events, trauma, chronic illness, certain medications, and hormonal shifts (after childbirth, during menopause) can all trigger episodes. In many cases, it’s a combination: someone with a genetic predisposition encounters enough stress or biological change to tip the balance.
MDD vs. Persistent Depressive Disorder
People sometimes confuse MDD with persistent depressive disorder (PDD), formerly called dysthymia. The key difference is duration and intensity. MDD involves distinct episodes with more acute, severe symptoms lasting at least two weeks. PDD is a lower-grade depression that lingers for at least two years in adults, with symptoms present most days. Someone with PDD might describe themselves as “always kind of down” rather than experiencing the dramatic drops that characterize MDD.
PDD requires only two symptoms beyond depressed mood (like low energy and poor self-esteem), while MDD requires five. It’s also possible to have both simultaneously, a situation sometimes called “double depression,” where a chronic low mood is punctuated by full depressive episodes.
How Severity Is Measured
Clinicians often use a screening tool called the PHQ-9, a nine-question survey that maps directly onto the diagnostic criteria. Each question is scored from 0 to 3 based on how often you’ve experienced a symptom over the past two weeks. The total score falls into four ranges: 5 to 9 indicates mild depression, 10 to 14 moderate, 15 to 19 moderately severe, and 20 to 27 severe. Many primary care doctors use this as a starting point before referring to a mental health specialist. You can find versions of the PHQ-9 online, though a score on a questionnaire isn’t the same as a diagnosis.
Treatment Options
For adults, the American Psychological Association recommends either psychotherapy or antidepressant medication as a first-line treatment, with the choice made collaboratively between patient and clinician. Several types of therapy have comparable effectiveness: cognitive behavioral therapy (CBT), interpersonal therapy, psychodynamic therapy, and supportive therapy all show strong results. When combining therapy and medication, CBT or interpersonal therapy paired with an SSRI or SNRI (the two most commonly prescribed classes of antidepressants) is the recommended approach.
For adolescents, the guidelines are more specific. CBT and a version of interpersonal therapy adapted for teens are the recommended psychotherapies. If medication is needed, fluoxetine is the first choice over other antidepressants in this age group. For children under 12, there isn’t yet enough evidence to firmly recommend any single therapy or medication, so treatment is more individualized.
For older adults, group-based approaches like cognitive behavioral therapy or life review therapy are recommended, and combining medication with interpersonal therapy tends to outperform therapy alone.
When Standard Treatments Don’t Work
Some people don’t respond adequately to first-line options, a situation called treatment-resistant depression. Electroconvulsive therapy (ECT) is one of the most studied alternatives. Clinical trials have historically cited remission rates of 70% to 90%, but real-world results are more modest. In community hospital settings, remission rates range from about 30% to 47%, with response rates (meaningful improvement, even if not full remission) between 50% and 60% for people who haven’t responded to medication. ECT is done under general anesthesia and typically involves multiple sessions over several weeks. It’s not a last resort in the dramatic sense it’s sometimes portrayed, but it is usually reserved for severe or medication-resistant cases.
Relapse and Long-Term Outlook
One of the most important things to understand about MDD is that it tends to come back. After a first episode, roughly half of all patients will experience a relapse. After a second episode, that risk climbs to about 70%. After a third, it reaches 90%. Of those who relapse, the majority (79%) do so within the first six months after completing treatment.
Each successive episode also tends to be harder to treat, with greater severity and more resistance to the interventions that worked previously. This is why ongoing care after remission matters so much. Continuing therapy, staying on medication for a recommended period rather than stopping as soon as you feel better, and building relapse-prevention strategies can meaningfully change the long-term trajectory of the disorder.
Warning Signs of a Crisis
MDD can become a medical emergency when it involves thoughts of suicide. These thoughts exist on a spectrum, from passive wishes like “I’d be fine if I just didn’t wake up” to active plans about self-harm. Both warrant immediate attention. Other warning signs include sudden withdrawal from friends and family, increasing hopelessness, agitation, giving away possessions, or a growing preoccupation with death. If someone expresses a desire to harm or kill themselves, that is an emergency, not a phase to wait out. The 988 Suicide and Crisis Lifeline (call or text 988 in the U.S.) provides immediate support around the clock.