What is depression?
Depression — clinically known as major depressive disorder (MDD) — is a serious medical condition characterised by persistent low mood, loss of interest or pleasure in activities, and a constellation of physical, cognitive, and emotional symptoms that significantly impair a person's ability to function in daily life.
It is important to understand what depression is not. It is not ordinary sadness, not a character weakness, not self-indulgence, and not something that can be resolved through willpower alone. Depression involves measurable changes in brain structure, chemistry, inflammatory pathways, and neural circuit functioning. It is as biological as diabetes or hypertension — and just as deserving of compassionate, evidence-based treatment.
The defining feature of a major depressive episode is that it must last at least two weeks and represent a change from previous functioning. Symptoms must cause significant distress or impairment — they are not a normal response to grief, though grief and depression can coexist and grief can trigger clinical depression in vulnerable individuals.
Brain imaging studies show measurable structural changes in people with depression — reduced volume in the hippocampus and prefrontal cortex, altered connectivity between the amygdala and prefrontal regions, and dysregulated activity in the default mode network. Inflammatory markers (IL-6, TNF-α, CRP) are elevated in many depressed patients. Chronic depression is associated with accelerated cellular ageing. These findings make clear that depression is not a mental attitude problem — it is a whole-body illness with profound neurobiological underpinnings.
Types of depression
🧠 Major Depressive Disorder (MDD)
- Discrete depressive episodes ≥2 weeks
- Most common form of clinical depression
- Single episode or recurrent
- Can be mild, moderate, or severe
- Excellent response to treatment
- Specifiers: melancholic, anxious, psychotic
⏳ Persistent Depressive Disorder (PDD)
- Formerly called dysthymia
- Chronic low mood ≥2 years
- Fewer symptoms than MDD but unrelenting
- "Double depression": MDD episode on PDD
- Often underdiagnosed ("I've always been like this")
- Responds to same treatments as MDD
🔄 Bipolar Depression
- Depressive episodes within bipolar disorder
- Types I and II (hypomanic/manic poles)
- Critical to distinguish from MDD
- Antidepressants alone can trigger mania
- Requires mood stabilisers as foundation
- Family history of bipolar is key clue
☀️ Seasonal Affective Disorder (SAD)
- Seasonal pattern — usually winter onset
- Linked to reduced light exposure
- Atypical features: hypersomnia, carb craving
- Light therapy: 10,000 lux for 30 min each morning
- SSRIs also effective
- Summer SAD (rarer) has different features
👶 Postpartum Depression (PPD)
- Onset within 4 weeks of delivery (DSM-5)
- Affects ~15% of new mothers
- Distinct from "baby blues" (days 2–5)
- Fathers and non-birthing parents also affected
- Zuranolone (Zurzuvae): first PPD-specific oral treatment
- Early treatment protects mother–infant bonding
🔴 PMDD
- Premenstrual Dysphoric Disorder
- Severe mood symptoms in luteal phase
- Resolves within days of menstruation
- Not "just PMS" — severely impairing
- Responds well to SSRIs (continuous or luteal)
- Hormonal interventions also used
Symptoms: the full picture
The DSM-5 diagnostic criteria for major depressive disorder require at least five of the following nine symptoms present during the same 2-week period, representing a change from previous functioning — and must include either depressed mood or loss of interest/pleasure:
| # | Symptom | How it often presents | Severity marker |
|---|---|---|---|
| 1 | Depressed mood | Persistent sadness, emptiness, hopelessness; may be irritability in children/teens | Present most of the day, nearly every day |
| 2 | Anhedonia | Loss of interest or pleasure in previously enjoyed activities — hobbies, social interaction, sex | Core symptom; required for diagnosis with #1 |
| 3 | Weight/appetite change | Significant weight loss or gain (>5% in a month); appetite increase or decrease | Not due to dieting |
| 4 | Sleep disturbance | Insomnia (difficulty falling or staying asleep; early morning waking) or hypersomnia | Nearly every night |
| 5 | Psychomotor changes | Observable slowing (psychomotor retardation) or agitation — not just subjective restlessness | Observable by others |
| 6 | Fatigue / energy loss | Profound exhaustion; even small tasks feel overwhelming; "getting out of bed is like climbing Everest" | Nearly every day |
| 7 | Worthlessness / guilt | Excessive or inappropriate guilt; feeling like a burden; distorted self-criticism | Not merely self-reproach about being ill |
| 8 | Cognitive impairment | Difficulty concentrating, thinking clearly, or making decisions — "brain fog" | Diminished ability to function at work/study |
| 9 | Suicidal ideation | Recurrent thoughts of death, passive wishes to die, suicidal ideation with or without a plan | Requires immediate safety assessment |
Call 911 or go to your nearest emergency department. Stay with the person. Remove access to means if possible. Call or text 988 (Suicide & Crisis Lifeline) for immediate support.
Depression across different groups
Depression presents differently depending on age, sex, and life circumstances. Recognising these variations prevents missed diagnoses.
Depression in men
Men are significantly less likely to be diagnosed with depression — not because they experience it less, but because they present differently and are less likely to seek help. Men with depression are more likely to show: irritability and anger rather than sadness, risk-taking behaviour, increased alcohol or substance use, physical complaints (headache, digestive problems), and withdrawal from relationships. Men die by suicide at 3–4 times the rate of women. This is a public health crisis that requires reducing stigma and expanding the clinical lens for how depression manifests.
Depression in older adults
Depression in older adults is frequently underdiagnosed, misattributed to "normal ageing," or masked by physical comorbidities. Key features include: more prominent cognitive symptoms (mimicking early dementia — "pseudodementia"), somatic complaints, social withdrawal, and loss of motivation rather than explicit sadness. Late-life depression also responds to the same treatments as depression at other ages, though medication titration is slower and vascular contributions to mood must be considered.
Depression in children and adolescents
In young people, depressed mood may manifest as irritability, unexplained physical complaints, school refusal, social withdrawal, or declining academic performance rather than the adult presentation of overt sadness. Adolescents are at particularly high risk — rates of depression have risen significantly since 2012, with social media exposure, academic pressure, and reduced sleep cited as contributing factors. Early intervention is critical; untreated adolescent depression increases lifetime risk of recurrence and substance use disorders.
Causes and neuroscience
Depression does not have a single cause. It arises from the interaction of genetic vulnerability with biological, psychological, and social factors. The old "chemical imbalance" (serotonin deficiency) model is an oversimplification that does not capture the complexity of what we now know about depressive neurobiology.
Biological factors
- Genetics: Depression is 37–40% heritable. Having a first-degree relative with MDD roughly doubles your risk. Dozens of genetic variants contribute small individual effects.
- Neuroinflammation: Elevated inflammatory cytokines (IL-6, TNF-α, IL-1β) are found in a significant subset of depressed patients. This explains why inflammatory conditions (autoimmune disease, obesity, chronic pain) are strong risk factors, and why anti-inflammatory treatments show promise in this subgroup.
- HPA axis dysregulation: Chronic stress dysregulates the hypothalamic-pituitary-adrenal axis, leading to abnormal cortisol patterns. Elevated cortisol damages the hippocampus and impairs neurogenesis.
- Neuroplasticity: Reduced BDNF (brain-derived neurotrophic factor) in depression impairs synaptic plasticity and neurogenesis in the hippocampus. Most antidepressants — and exercise — increase BDNF, which may be a core mechanism of therapeutic effect.
- Network dysregulation: Modern neuroimaging identifies depression as a disorder of neural circuit dysregulation — particularly overactivity in the default mode network (ruminative thought) and impaired connectivity between prefrontal regulatory areas and the amygdala.
Psychological and social factors
- Adverse childhood experiences (ACEs): Physical, emotional, or sexual abuse; neglect; parental mental illness or substance use — all significantly increase adult depression risk through epigenetic and neurobiological mechanisms.
- Cognitive style: Negative attribution patterns — the tendency to interpret events in systematically negative ways — both predict and perpetuate depression. This is the cognitive model that underpins CBT.
- Life events: Loss, relationship breakdown, unemployment, trauma, and chronic stress are potent triggers in genetically and biologically vulnerable individuals.
- Social isolation: Loneliness is as powerful a risk factor for depression (and early mortality) as smoking 15 cigarettes per day. Social connection is a fundamental neurobiological need.
Diagnosis and screening
Depression is diagnosed clinically — through structured interview and validated symptom measures. There is no blood test for depression, though investigations are useful to exclude medical causes of depressive symptoms.
The PHQ-9
The Patient Health Questionnaire-9 (PHQ-9) is the most widely used validated screening and severity tool. Nine questions map directly to the DSM-5 criteria, each scored 0–3 (not at all / several days / more than half the days / nearly every day). Total score interpretation: 0–4 minimal; 5–9 mild; 10–14 moderate; 15–19 moderately severe; 20–27 severe. The PHQ-2 (questions 1 and 2 only — depressed mood and anhedonia) is used as a rapid initial screen.
Medical causes to exclude
Several medical conditions can cause or mimic depressive symptoms and must be excluded — especially in first presentations or atypical features:
- Hypothyroidism (TSH, free T4) — very common; easily treated
- Anaemia (FBC, iron studies, B12, folate)
- Diabetes and metabolic syndrome
- Chronic pain conditions
- Sleep apnea — severely underrecognised cause of fatigue and low mood
- Chronic infections (hepatitis C, HIV, Lyme disease)
- Neurological conditions (early Parkinson's, MS, post-stroke)
- Medications: beta-blockers, isotretinoin, corticosteroids, interferon, some hormonal contraceptives
- Substance use and withdrawal (alcohol, cannabis, stimulants)
Psychotherapy options
Psychotherapy is as effective as antidepressants for mild-to-moderate depression, and the combination of therapy plus medication outperforms either alone for moderate-to-severe depression. Crucially, psychotherapy produces lasting structural changes in brain function — its benefits extend beyond the treatment period, reducing relapse risk.
| Therapy | Core approach | Evidence | Best suited for | Format |
|---|---|---|---|---|
| Cognitive Behavioural Therapy (CBT) | Identifies and challenges distorted thinking patterns; builds behavioural activation | Most researched; strong for MDD, anxiety, OCD, PTSD | First-line for most depression; especially cognitive features | 12–20 sessions; also effective online |
| Behavioural Activation (BA) | Focuses purely on increasing engagement with rewarding activities; breaks avoidance-depression cycle | Equivalent to full CBT for depression; simpler to deliver | Anhedonia; social withdrawal; those who prefer action over thought analysis | 8–16 sessions; accessible format |
| Interpersonal Therapy (IPT) | Addresses interpersonal problems — grief, role transitions, disputes, social deficits — that maintain depression | Strong evidence especially for postpartum depression; grief-related depression | Depression linked to relationship issues, life transitions, bereavement | 12–16 sessions |
| Psychodynamic Therapy | Explores unconscious patterns, early relationships, and how past experiences shape current mood | Good evidence for longer-term depression; growing evidence base | Recurrent depression; personality factors; complex trauma history | Longer-term; 16–40+ sessions |
| MBCT (Mindfulness-Based Cognitive Therapy) | Combines mindfulness meditation with CBT elements; teaches decentering from depressive thoughts | NICE-recommended for recurrent MDD; reduces relapse by 44% in those with 3+ episodes | Recurrent depression; preventing relapse; residual symptoms | 8-week group programme |
| Problem-Solving Therapy (PST) | Structured approach to identifying and solving life problems that contribute to depression | Good evidence; particularly effective in older adults and primary care | Depression in context of concrete life problems; pragmatic patients | 6–12 sessions |
Antidepressants explained
Antidepressants are effective medications — not "happy pills" — that restore normal function to dysregulated neural circuits. They typically take 2–6 weeks to produce full therapeutic effect, and the first medication trialled is not always the right one. Persistence and close communication with your prescriber are key.
Selective serotonin reuptake inhibitors (SSRIs) are the first-line antidepressants for most patients — well-tolerated, widely available, and effective across a broad range of presentations including depression, anxiety disorders, OCD, PTSD, and PMDD.
| SSRI | Usual dose | Key features | Common side effects |
|---|---|---|---|
| Sertraline (Zoloft) | 50–200 mg/day | Most widely prescribed; good tolerability; safe in cardiac disease | GI upset initially; sexual dysfunction; insomnia |
| Escitalopram (Lexapro) | 10–20 mg/day | Very well tolerated; fewest drug interactions; once daily | Sexual dysfunction; nausea; headache |
| Fluoxetine (Prozac) | 20–60 mg/day | Long half-life (easier to stop); activating; approved for adolescents | Insomnia; anxiety initially; prolonged half-life means slow washout |
| Citalopram (Celexa) | 20–40 mg/day | Simple pharmacology; inexpensive | QTc prolongation at higher doses — dose-capped at 40 mg |
| Paroxetine (Paxil) | 20–50 mg/day | Sedating (may help insomnia); good for anxiety/panic | Most anticholinergic; weight gain; worst discontinuation syndrome; avoid in pregnancy |
| Fluvoxamine (Luvox) | 100–300 mg/day | First-line for OCD; multiple daily doses often required | Significant drug interactions via CYP1A2; sedation; nausea |
Serotonin-norepinephrine reuptake inhibitors (SNRIs) block reuptake of both serotonin and norepinephrine. The dual mechanism may offer advantages for pain comorbidity and some patients who don't respond to SSRIs.
| SNRI | Usual dose | Key features | Notes |
|---|---|---|---|
| Venlafaxine (Effexor) | 75–225 mg/day | More norepinephrine at higher doses; effective for anxiety and neuropathic pain; hot flashes | Blood pressure elevation at high doses; significant discontinuation syndrome; use XR formulation |
| Duloxetine (Cymbalta) | 60–120 mg/day | FDA-approved for depression, GAD, fibromyalgia, diabetic neuropathy, chronic musculoskeletal pain | Nausea is common; hepatic metabolism; avoid in heavy alcohol use |
| Desvenlafaxine (Pristiq) | 50 mg/day | Active metabolite of venlafaxine; simpler dosing; fewer drug interactions | Fixed dose (less titration flexibility) |
| Levomilnacipran (Fetzima) | 40–120 mg/day | Greater norepinephrine selectivity; may be more activating; targets fatigue and motivation | Increased heart rate; urinary hesitancy; avoid in uncontrolled narrow-angle glaucoma |
| Drug | Mechanism | Key advantages | Key cautions |
|---|---|---|---|
| Bupropion (Wellbutrin) | NDRI (norepinephrine-dopamine reuptake inhibitor) | No sexual dysfunction; weight neutral/loss; helps smoking cessation; activating | Lowers seizure threshold; avoid in eating disorders/seizure history; insomnia; no help for anxiety |
| Mirtazapine (Remeron) | NaSSA — blocks α2 receptors; antihistaminergic | Excellent for insomnia and poor appetite; sedating; few sexual side effects; fast onset | Significant weight gain; sedation; metabolic effects; paradoxically less sedating at higher doses |
| Vortioxetine (Trintellix) | Serotonin modulator and stimulator (SMS) | Cognitive benefits (processing speed, executive function); well-tolerated | More expensive; nausea; sexual dysfunction less than SSRIs |
| Vilazodone (Viibryd) | SSRI + partial 5-HT1A agonist | Fewer sexual side effects than traditional SSRIs | Must be taken with food; GI side effects; drug interactions |
| TCAs (Amitriptyline, Nortriptyline) | Non-selective monoamine reuptake inhibition | Effective; useful for pain, migraine prophylaxis, insomnia; inexpensive | Cardiotoxic in overdose; anticholinergic; sedation; not first-line |
| MAOIs (Phenelzine, Tranylcypromine) | Monoamine oxidase inhibition | Highly effective, especially atypical depression; last resort | Severe dietary restrictions (tyramine); multiple drug interactions; rarely used |
A new generation of fast-acting, mechanism-novel treatments is transforming options for treatment-resistant and severe depression — moving beyond the monoamine paradigm entirely.
| Treatment | Mechanism | Status | Key facts |
|---|---|---|---|
| Ketamine IV | NMDA glutamate receptor antagonist | FDA-approved; widely available at clinics | Rapid antidepressant effect within hours; 6 infusions over 3 weeks typical; response rate ~60–70% in TRD; dissociative side effects during infusion; not covered by most insurance |
| Esketamine (Spravato) | S-enantiomer of ketamine; intranasal | FDA-approved for TRD and MDD with acute suicidality | Self-administered under clinical supervision; given twice weekly for 4 weeks then tapered; must remain in clinic 2 hours post-dose; covered by many insurers |
| Zuranolone (Zurzuvae) | GABAA positive allosteric modulator (neuroactive steroid) | FDA-approved Aug 2023 — first oral fast-acting antidepressant | 14-day oral course produces rapid improvement (day 3); FDA-approved for MDD and PPD; taken once nightly; sedation main side effect |
| Psilocybin-assisted therapy | 5-HT2A agonist; promotes neuroplasticity | Phase 3 trials; breakthrough therapy designation; FDA approval anticipated 2025–2026 | 2–3 supervised sessions; durable effects (remission maintained 12+ weeks in trials); COMP360 and other compounds in late-stage trials; profound paradigm shift in psychiatry |
| TMS (Transcranial Magnetic Stimulation) | Magnetic pulses stimulate DLPFC | FDA-approved; widely covered by insurance | Daily sessions for 4–6 weeks; no anaesthesia; minimal side effects; iTBS accelerated protocol possible; response ~50–60% in TRD |
| ECT (Electroconvulsive Therapy) | Electrically induced seizure under anaesthesia | Most effective treatment for severe/TRD depression | Response rates 70–90% in severe MDD; safe and effective despite stigma; used for catatonia, psychotic depression, severe suicidality; memory side effects manageable with modern technique |
Frequently asked questions
Untreated major depressive episodes typically last 6–12 months. About 20% of episodes last more than 2 years. With appropriate treatment, most people begin to feel better within 4–8 weeks. Without treatment, depression tends to recur — each episode slightly increases the risk of subsequent ones and can lead to structural brain changes that make future episodes more frequent and harder to treat. This is why early, adequate treatment matters so much.
Sadness is a normal, healthy emotion — a proportionate, time-limited response to loss or difficulty that does not impair overall functioning. Clinical depression is a medical condition characterised by persistent low mood lasting at least 2 weeks plus multiple physical and cognitive symptoms. Depression is not a character failing or an attitude problem. People with depression cannot resolve it by "pulling themselves together" — any more than someone with a broken leg can resolve it by trying harder to walk. It requires appropriate medical and/or psychological treatment.
Many people achieve full recovery and never experience another episode. However, depression has a tendency to recur — about 50% of people who have one episode will have a second, and 80% of those who have had two will have a third. Maintenance antidepressant therapy (continuing medication for 2+ years after remission in high-risk patients) significantly reduces relapse risk. Mindfulness-based cognitive therapy (MBCT) also reduces relapse by approximately 44% in patients with three or more prior episodes.
Antidepressants are not addictive in the classical sense — they do not cause drug-seeking behaviour, tolerance requiring dose escalation, or euphoria. However, SSRIs and SNRIs can cause discontinuation syndrome if stopped abruptly — symptoms include dizziness, flu-like feelings, electric shock sensations ("brain zaps"), anxiety, and irritability. These are managed by gradually tapering the dose under medical supervision. This is physical dependence, not addiction — an important distinction. Always make decisions about stopping antidepressants in partnership with your prescribing doctor.
- World Health Organization. Depression fact sheet. 2023. Available from: who.int/news-room/fact-sheets/detail/depression
- American Psychiatric Association. Diagnostic and Statistical Manual of Mental Disorders, 5th Edition Text Revision (DSM-5-TR). 2022.
- Cipriani A, et al. Comparative efficacy and acceptability of 21 antidepressant drugs for the acute treatment of adults with major depressive disorder. Lancet. 2018;391(10128):1357–1366.
- Patel V, et al. The Lancet Commission on global mental health and sustainable development. Lancet. 2018;392(10157):1553–1598.
- Carhart-Harris R, et al. Trial of Psilocybin versus Escitalopram for Depression. NEJM. 2021;384(15):1402–1411.
- Murrough JW, et al. Antidepressant Efficacy of Ketamine in Treatment-Resistant Major Depression. Am J Psychiatry. 2013;170(10):1134–1142.
- Segel M, Bhatt D, Bhatt D. Zuranolone for Major Depressive Disorder. NEJM. 2023;389:1183–1185.
- Kuyken W, et al. Efficacy of Mindfulness-Based Cognitive Therapy in Prevention of Depressive Relapse (PREVENT). Lancet. 2016;386(9988):63–73.