What is Depression?
Depression, or major depressive disorder, is a medical condition involving persistently low mood or loss of interest and pleasure for at least two weeks, along with changes in sleep, appetite, energy, concentration, and self-worth. It is common and treatable. Psychotherapy, medication, and lifestyle approaches all have evidence, and most people improve with appropriate care.
Key takeaways
- Depression is a medical condition, not a character flaw or a lack of willpower.
- Diagnosis requires symptoms most of the day, nearly every day, for at least two weeks, with functional impact.
- The PHQ-9 is a widely used screening and monitoring questionnaire, but it does not replace clinical assessment.
- Psychotherapy and antidepressant medication are both effective, and combining them often works best for moderate to severe depression.
- Antidepressants typically take four to six weeks for full effect, which is a common reason people stop too early.
Overview
Depression is among the most common medical conditions in the world and a leading cause of disability. It is far more than sadness. The two core features are persistently low mood and anhedonia — a loss of interest or pleasure in things that were previously enjoyable — and these are accompanied by changes across sleep, appetite, energy, concentration, movement, and self-worth. To meet criteria, symptoms must be present most of the day, nearly every day, for at least two weeks, and must interfere with functioning.
It is also one of the most treatable conditions in medicine, which makes the persistent stigma around it particularly costly. Effective options range from structured psychotherapy to several medication classes to brain stimulation for treatment-resistant cases. What is often underappreciated is how physical depression can be: unexplained fatigue, pain, and appetite change are common presentations, and in older adults, memory complaints and withdrawal may be more prominent than reported sadness.
What Depression is
Major depressive disorder is diagnosed when at least five of nine symptoms are present for two weeks or more, including either depressed mood or loss of interest. The nine cover mood, interest, appetite or weight change, sleep disturbance, psychomotor change, fatigue, worthlessness or guilt, concentration difficulty, and thoughts of death or suicide.
Related conditions share the territory: persistent depressive disorder describes lower-grade depression lasting two years or more; seasonal patterns recur at particular times of year; perinatal depression occurs during pregnancy or after birth; and depressive episodes within bipolar disorder require a different treatment approach, which is why clinicians ask about periods of unusually elevated mood or energy before prescribing.
Common symptoms
Symptoms span mood, body, and thinking, and their combination varies considerably between individuals.
- Persistently low, empty, or irritable mood most of the day
- Loss of interest or pleasure in activities that used to matter
- Fatigue or loss of energy that rest does not fix
- Sleep disturbance — difficulty sleeping, or sleeping much more than usual
- Appetite or weight change in either direction
- Difficulty concentrating, remembering, or making decisions
- Feelings of worthlessness, excessive guilt, or self-blame
- Slowed movement and speech, or restlessness and agitation
Less common symptoms
- Physical symptoms with no other explanation — headaches, digestive problems, or diffuse pain
- Thoughts of death, or thoughts of harming yourself
- Memory and attention problems prominent enough to resemble dementia in older adults
- Loss of libido
- Anxiety or panic occurring alongside low mood
- Psychotic features such as hearing voices or fixed guilty beliefs — uncommon and requiring urgent care
Risk factors
- Previous episode of depression — the strongest single predictor of another
- Family history of depression or bipolar disorder — a genetic contribution is well established
- Chronic medical illness — diabetes, heart disease, stroke, cancer, and chronic pain all raise risk substantially
- Adverse childhood experiences and trauma
- Recent loss, relationship breakdown, unemployment, or financial strain
- Social isolation and loneliness
- Alcohol or other substance use — both a risk factor and a common consequence
- Hormonal transitions — postpartum period, perimenopause, and thyroid disease
- Certain medicines, including some corticosteroids and interferons — worth reviewing with a clinician
Causes
Depression has no single cause. The best-supported model is biopsychosocial: genetic susceptibility interacts with life circumstances, chronic stress, and the physical health of the brain and body. Changes in neurotransmitter signalling, stress hormone regulation, inflammation, and neuroplasticity have all been demonstrated, but none is a complete explanation, and the older idea that depression is simply a serotonin deficiency is an oversimplification that the evidence does not support.
Medical contributors are worth excluding because they are treatable in their own right: thyroid dysfunction, vitamin B12 deficiency, obstructive sleep apnea, anemia, and certain medicines can all produce or worsen depressive symptoms.
How it is diagnosed
Diagnosis is clinical, supported by structured questionnaires and by tests that exclude medical contributors.
| Test or assessment | What it looks at |
|---|---|
| Clinical interview | Assesses symptoms, duration, functional impact, safety, substance use, and history of elevated mood, which distinguishes bipolar from unipolar depression. |
| PHQ-9 questionnaire | A nine-item scale used widely for screening and for tracking response over time. It quantifies severity but does not diagnose on its own. |
| Risk assessment | Direct questions about thoughts of self-harm or suicide, plans, and access to means. Asking does not increase risk and is a standard part of care. |
| Thyroid function tests | Hypothyroidism commonly mimics depression and is easily treated, so it is routinely checked. |
| Blood count, vitamin B12, and vitamin D | Anemia and B12 deficiency can produce fatigue and low mood; testing is guided by the clinical picture. |
| Screening for coexisting conditions | Anxiety, substance use, sleep apnea, and chronic pain frequently accompany depression and change the treatment plan. |
Treatment overview
Treatment is matched to severity, preference, and history, and often combines approaches. The categories below describe what is available; the choice of therapy or medicine and any dosing is set with a clinician.
Psychotherapy
Cognitive behavioural therapy, behavioural activation, interpersonal therapy, and problem-solving therapy all have strong evidence. Availability includes in-person, group, and validated digital formats.
Antidepressant medication classes
SSRIs and SNRIs are typical starting classes, with bupropion, mirtazapine, and others used depending on symptoms and side-effect profile. Full effect usually takes four to six weeks.
Combination treatment
Psychotherapy plus medication generally outperforms either alone for moderate to severe depression.
Behavioural and lifestyle interventions
Structured exercise has genuine antidepressant effect sizes in trials; sleep regulation, social contact, and reducing alcohol also matter.
Treatment-resistant options
Augmentation strategies, repetitive transcranial magnetic stimulation, esketamine, and electroconvulsive therapy for severe or refractory depression, delivered under specialist care.
Light therapy
Effective for depression with a seasonal pattern, and sometimes used as an adjunct in non-seasonal depression.
Collaborative care and support
Care coordination, peer support, and involving family or friends improve engagement and outcomes, particularly in primary care settings.
Lifestyle considerations
- Build in regular physical activity — the evidence for exercise as a treatment component is genuinely strong.
- Keep a consistent sleep and wake schedule; disturbed sleep both causes and worsens depression.
- Plan small, specific, achievable activities rather than waiting to feel motivated — motivation usually follows action.
- Maintain social contact even when the urge is to withdraw.
- Reduce alcohol, which is a depressant and interacts with most treatments.
- Get outside in daylight, especially in the morning.
- Eat regularly; skipping meals worsens energy and concentration.
- Tell someone you trust what you are going through, and let them help with practical things.
Prevention
- Treat a first episode fully rather than stopping early — incomplete recovery predicts recurrence.
- Maintain regular exercise, sleep, and social connection, all of which are protective.
- Address chronic stress, and seek support during major life transitions.
- Screen for and treat perinatal depression; prevention programs exist for those at higher risk.
- Manage chronic medical conditions and pain, which substantially raise depression risk.
- Limit alcohol and other substances.
- Keep follow-up appointments during recovery, when relapse risk is highest.
Warning signs and when to get help
Signs that need emergency care
If you believe you are experiencing a medical emergency, call emergency services immediately (911 in the United States).
- Thoughts of ending your life, a plan, or access to means — call or text 988 for the Suicide and Crisis Lifeline in the US, or call emergency services
- Thoughts of harming someone else
- Hearing voices, or fixed beliefs that are not based in reality
- Being unable to eat, drink, or care for yourself
- A serious act of self-harm — call emergency services immediately
Signs that warrant a prompt appointment
These are not emergencies, but they are worth a call to your clinician rather than a wait-and-see.
- Symptoms lasting more than two weeks or interfering with work, study, or relationships
- Worsening despite treatment, or new agitation or restlessness after starting a medicine
- Increasing alcohol or substance use
- New periods of unusually high energy, reduced need for sleep, or impulsive behaviour
- Postpartum low mood, intrusive thoughts, or difficulty bonding with your baby
Questions to ask a healthcare professional
Take these to your next appointment — or build an agenda with the Appointment Prep tool.
- What type of depression do you think I have, and how severe is it?
- What are the pros and cons of therapy versus medication for me?
- How long before I should expect to feel a difference?
- What side effects are common early on, and which ones should I call about?
- How long would I need to stay on treatment after I feel better?
- What therapy options are available to me locally or online?
- Should thyroid, B12, or sleep apnea be checked?
- What is my plan if things get worse, and who do I contact out of hours?
Frequently asked questions
How long do antidepressants take to work?
Some improvement in sleep, appetite, or energy may appear within one to two weeks, but the full effect usually takes four to six weeks. Stopping early because "it is not working" is one of the most common reasons treatment fails. Discuss any change with your prescriber rather than stopping abruptly.
Is depression caused by a chemical imbalance?
That phrase oversimplifies things. Neurotransmitter signalling is involved, but so are stress hormones, inflammation, neuroplasticity, genetics, and life circumstances. Antidepressants work for many people without depression being a simple deficiency of any one chemical.
Can exercise really treat depression?
Yes — structured exercise has meaningful effect sizes in randomised trials, particularly for mild to moderate depression, and it is a legitimate part of treatment rather than just general advice. It works best alongside, not instead of, other treatment for more severe depression.
Will I need to take medication forever?
Not usually. After a first episode, treatment is commonly continued for six to twelve months after recovery to reduce relapse. People with recurrent episodes may benefit from longer-term treatment. Any change should be planned and tapered with your prescriber.
What is the difference between sadness and depression?
Sadness is a normal response to loss or difficulty, fluctuates, and generally allows some enjoyment. Depression is persistent most of the day for at least two weeks, affects sleep, appetite, energy, and self-worth, and interferes with functioning — often without a clear trigger.
Where can I get help right now?
In the US, call or text 988 to reach the Suicide and Crisis Lifeline, available 24 hours a day. If you are in immediate danger, call emergency services. You can also contact your primary care clinician, who can start assessment and treatment.
Sources
- NIMH — Depression — NIH overview of symptoms, causes, and treatments
- NAMI — Depression — Patient and family support information
- CDC — Mental Health — US public health data and resources
- MedlinePlus — Depression — Consumer summary and treatment links
- 988 Suicide & Crisis Lifeline — Free, confidential crisis support 24/7 in the US
Health Captain links to primary public-health and clinical sources so you can read the original material yourself. See our sources policy.