Depression: symptoms, causes and treatment

Key points

• Depression is low mood or loss of interest lasting weeks or months and disrupting daily life.
• In England, 3.8% of adults met the criteria for a depressive episode in 2023/4 [3].
• NICE splits depression into less severe and more severe, at a PHQ-9 score of 16 [1].
• For less severe depression, NICE says not to offer antidepressants routinely as first treatment [1].
• The low serotonin account is not supported in that simple form, though antidepressants still help many people [8][10].

What depression is

Depression is a mental health condition in which persistent low mood, or loss of interest and enjoyment, lasts weeks or months and interferes with everyday life. The NHS calls it more than unhappiness, and not something a person can snap out of [2]. In the Adult Psychiatric Morbidity Survey 2023/4, 3.8% of adults in England met the criteria for a depressive episode: 4.5% of women, 3.1% of men [3].

If you need help now. Samaritans: call 116 123, free, 24/7. Shout: text 85258. NHS 111, then choose the mental health option. Call 999 if anyone is in immediate danger.

Symptoms

Depression affects thinking, the body and social life at once, building over weeks rather than days [2].

Psychological. Continuous low mood; feeling hopeless or helpless; low self-esteem, guilt, tearfulness or irritability; no motivation or enjoyment; difficulty making decisions; anxiety; thoughts of self-harm or suicide [2].

Physical. Low energy; moving or speaking more slowly than usual; disturbed sleep, often trouble falling asleep or waking very early; appetite or weight changes; unexplained aches and pains; reduced sex drive [2].

Social. Avoiding friends, doing fewer activities, dropping hobbies, and difficulties at home, at work or in the family [2].

It is commonly confused with grief, anxiety disorders and an underactive thyroid [2], and with bipolar disorder, which is why NICE asks clinicians to check for past raised mood [1].

Types

Depression is recognised in several forms.

• Major depressive disorder, also called a depressive episode or clinical depression. This is the standard diagnosis, graded by NICE as less severe or more severe [1].
• Persistent depressive disorder, or dysthymia. Less intense symptoms running on for years. NICE calls these chronic depressive symptoms, and notes many people with them never sought treatment [1].
• Seasonal affective disorder, depression following a seasonal pattern, usually starting in winter and easing in summer, though some have the reverse. Reduced daylight is thought to play a part [4].
• Postnatal depression, which can start in pregnancy, after birth or during the first year. It differs from the baby blues, which pass within two weeks, and commonly improves in three to six months [5].
• Premenstrual dysphoric disorder, a severe form of premenstrual syndrome in the days before a period. Symptoms resemble PMS but are far more intense, and can include feeling very anxious, angry or suicidal [6].

Depression within bipolar disorder is treated differently. A separate NICE guideline recommends therapy designed for bipolar disorder, and medicines such as quetiapine, rather than an antidepressant alone [7]. See our entry on bipolar disorder.

Causes and risk factors

Depression has no single cause. NICE asks clinicians to discuss contributing factors including past depression, other health conditions, relationship difficulties, past trauma, living conditions, alcohol and drug use, debt, unemployment and loneliness [1]. The NHS adds bereavement, relationship breakdown and job loss as triggers, and family history, childbirth and long-term physical illness as risks [2]. It can also start with no obvious trigger [2].

The common account, that depression is a chemical imbalance caused by low serotonin, does not hold up in that simple form. An umbrella review published online in 2022 in Molecular Psychiatry gathered the main strands of serotonin research and found no consistent evidence of an association between serotonin and depression [8]. It was contested: a comment signed by 36 researchers argued that it set aside relevant findings, that serotonin was never claimed to be the sole cause, and that antidepressants are effective for clinically significant depression [9].

Whether antidepressants work is a separate question. A network meta-analysis of 522 trials and 116,477 participants found all 21 antidepressants studied more effective than placebo in short-term treatment, differences between drugs being modest [10].

How it is diagnosed

There is no blood test or scan for depression. In England a GP usually makes the diagnosis, as can a psychiatrist, practitioner psychologist or other clinician competent to carry out a mental health assessment [1]. NICE suggests two identification questions: whether, in the last month, the person has often been bothered by feeling down or hopeless, and by having little interest or pleasure in doing things [1].

NICE says assessment should take in symptom severity, previous history, how long the episode has lasted and the degree of functional impairment, not a symptom count alone [1]. A validated measure such as the PHQ-9 is used to track treatment: NICE puts less severe depression below a score of 16, and more severe at 16 or above [1]. Clinicians ask directly about suicidal thoughts, and NICE is explicit that treatment should not be withheld because of assessed suicide risk [1].

A diagnosis names a pattern and opens a route to treatment. It does not predict how long an episode lasts.

Treatment

NICE matches treatment to severity and to the person’s preference, listing options in the order its committee judged effectiveness [1].

Less severe depression. Any option can be used first, but NICE says to consider the least intrusive first, which is guided self-help [1]. Its order is guided self-help, group cognitive behavioural therapy, group behavioural activation, individual CBT, individual behavioural activation, group exercise, group mindfulness and meditation, interpersonal psychotherapy, an SSRI antidepressant, counselling, short-term psychodynamic psychotherapy [1]. NICE says not to offer antidepressants routinely at this severity, only where that is the person’s informed preference [1]. Someone who declines treatment is offered active monitoring, reassessed within two to four weeks [1].

More severe depression. Any of these can be used first, in this order: individual CBT combined with an antidepressant, individual CBT, individual behavioural activation, antidepressant medication, individual problem-solving, counselling, short-term psychodynamic psychotherapy, interpersonal psychotherapy, guided self-help, group exercise [1].

NICE says SSRIs are generally well tolerated and the first choice for most people, and that antidepressants are typically continued for at least six months after symptoms settle [1]. They are reduced in stages, not stopped suddenly: withdrawal symptoms usually clear within one to two weeks, but can last several weeks or occasionally months [1]. Any change is for the prescriber.

If there is no response after four weeks of medication or four to six weeks of therapy, NICE says to review what is getting in the way, then consider switching therapy, adding an SSRI, changing medication or adding group exercise [1]. To prevent relapse it recommends group CBT or mindfulness-based cognitive therapy [1].

Living with depression

Depression pulls you out of the activities that lift mood, which lowers it further. Behavioural activation, one of the NICE-recommended treatments, works on that loop: you track the link between what you do and how you feel, then plan small activities back in before you feel like it [1].

• Physical activity counts as treatment, not an extra: group exercise appears in the NICE lists at both severities [1].
• Work and study can usually be adjusted. Occupational health, a manager or a university wellbeing service can change hours or deadlines, and a GP can issue a fit note.
• Before treatment ends, write down your warning signs and what you will do if they return. NICE treats this as part of therapy [1].

When to speak to a professional

See a GP if low mood or loss of interest has lasted more than two weeks, or is affecting your work, studies, relationships or ability to look after yourself [2]. NICE says depression services should have several ways in, including self-referral [1]. In England that is NHS Talking Therapies for anxiety and depression, which takes bookings without a GP referral.

Do not wait if you are having thoughts of suicide or self-harm. Call Samaritans free on 116 123 at any hour, text SHOUT to 85258, or call NHS 111 and choose the mental health option. Call 999 if anyone is in immediate danger. Saying so to a GP will not cost you treatment: NICE says it must not be withheld on the basis of suicide risk [1].

Common questions

How long does depression last?

There is no fixed length. NICE judges response at four weeks for medication and four to six weeks for therapy [1]. Postnatal depression commonly improves within three to six months [5]. Some people have one episode, others several.

Does everyone with depression need antidepressants?

No. For less severe depression NICE says not to offer them routinely, only where the person prefers it [1]. For more severe depression, medication, therapy and the two combined are all first-line options [1].

Is depression caused by a chemical imbalance in the brain?

The simple version is not supported. An umbrella review found no consistent evidence linking serotonin to depression [8], though other researchers argue it understated the evidence and stress that antidepressants remain effective [9].

What is the difference between depression and sadness?

Sadness is a response to something and it lifts. Depression persists for weeks or months, most of the day, and takes in energy, sleep, appetite and concentration as well as mood [2].

Clinical review pending

References

1. National Institute for Health and Care Excellence. Depression in adults: treatment and management (NG222), recommendations. NICE, 2022. https://www.nice.org.uk/guidance/ng222/chapter/recommendations
2. NHS. Depression in adults: overview, symptoms and causes. NHS. https://www.nhs.uk/mental-health/conditions/depression-in-adults/overview/ , https://www.nhs.uk/mental-health/conditions/depression-in-adults/symptoms/ , https://www.nhs.uk/mental-health/conditions/depression-in-adults/causes/
3. NHS England Digital. Adult Psychiatric Morbidity Survey: Survey of Mental Health and Wellbeing, England, 2023/4. Chapter 1: Common mental health conditions. NHS England, 2025. https://digital.nhs.uk/data-and-information/publications/statistical/adult-psychiatric-morbidity-survey/survey-of-mental-health-and-wellbeing-england-2023-24/common-mental-health-conditions
4. NHS. Overview: Seasonal affective disorder (SAD). NHS. https://www.nhs.uk/mental-health/conditions/seasonal-affective-disorder-sad/overview/
5. NHS. Feeling depressed after childbirth. NHS. https://www.nhs.uk/conditions/baby/support-and-services/feeling-depressed-after-childbirth/
6. NHS. Premenstrual syndrome (PMS). NHS. https://www.nhs.uk/conditions/pre-menstrual-syndrome/
7. National Institute for Health and Care Excellence. Bipolar disorder: assessment and management (CG185), recommendations. NICE, 2014, updated 2023. https://www.nice.org.uk/guidance/cg185/chapter/Recommendations
8. Moncrieff J, Cooper RE, Stockmann T, Amendola S, Hengartner MP, Horowitz MA. The serotonin theory of depression: a systematic umbrella review of the evidence. Molecular Psychiatry, 2023, volume 28, pages 3243 to 3256. https://www.nature.com/articles/s41380-022-01661-0
9. King’s College London. A response to “The serotonin theory of depression: a systematic umbrella review of the evidence”. KCL, 2023. https://www.kcl.ac.uk/news/a-response-to-the-serotonin-theory-of-depression-a-systematic-umbrella-review-of-the-evidence
10. Cipriani A, Furukawa TA, Salanti G, et al. Comparative efficacy and acceptability of 21 antidepressant drugs for the acute treatment of adults with major depressive disorder: a systematic review and network meta-analysis. The Lancet, 2018, volume 391, pages 1357 to 1366. https://www.thelancet.com/article/S0140-6736(17)32802-7/fulltext