In mild depression, low mood and loss of interest are the main symptoms; the sufferer may cry without any reason or be unresponsive to relatives and friends. In its more severe form, there is a loss of appetite; sleeping problems; lack of interest in and enjoyment of social activities; tiredness for no obvious reason; indifference to sexual activity; and a lack of concentration. The individual's physical and mental activities slow down and he or she may contemplate suicide. Symptoms may vary during the 24 hours, being less troublesome during the latter part of the day and worse at night. Some people get depressed during the winter months, probably a consequence of the long hours of darkness: this disorder – SEASONAL AFFECTIVE DISORDER SYNDROME, or SADS – is thought to be more common in populations living in areas with long winters and limited daylight. Untreated, a person with depressive symptoms may steadily worsen, even withdrawing to bed for much of the time, and allowing his or her personal appearance, hygiene and environment to deteriorate. Children and adolescents may also suffer from depression and the disorder is not always recognised, since they may complain of physical symptoms such as abdominal pain or headache, which diverts attention from the underlying problem.

A depressive illness rarely has a single obvious cause, although sometimes the death of a close relative, loss of employment or a broken personal relationship may trigger a bout. It probably has a genetic background; for instance, manic depression seems to run in some families. Viral infections sometimes cause depression and hormonal disorders – for example, HYPOTHYROIDISM or postnatal hormonal disturbances (postnatal depression) – may also cause it. Difficult family or social relations can contribute to the development of the disorder.

This depends on the type and severity of the depression.

The NATIONAL INSTITUTE FOR HEALTH AND CARE EXCELLENCE (NICE) recommends that, in mild depression, patients who are likely to get better spontaneously or who do not want intervention should simply be reassessed after a time. Antidepressant drugs are not recommended in this form of the condition. Rather doctors should advise a guided self-help programme based on COGNITIVE BEHAVIOUR THERAPY. Exercise programmes may also help. Short term psychological treatment, such as problem-solving therapy, brief CBT or counselling over about a 3 month period is advised for those with mild and moderate depression who request or need treatment. Antidepressant medication together with CBT may be recommended for some in this group, particularly if they do not respond to other procedures, and for all those who are diagnosed with severe depression. Where patients seem to pose a significant risk to themselves or others, referral to psychiatric services is usual. Antidepressant drugs are divided into three main groups: TRICYCLIC ANTIDEPRESSANT DRUGS (amitriptyline, imipramine and dothiepin are examples); MONOAMINE OXIDASE INHIBITORS (MAOIS) (phenelzine, isocarboxazid and tranylcypromine are examples, and in general these should be prescribed only by specialist mental health practitioners and general practitioners with a special interest in mental health); and SELECTIVE SEROTONIN-REUPTAKE INHIBITORS (SSRIS) (fluoxetine, fluvoxamine and paroxetine are examples). In routine care, an SSRI is used first as it is less likely to cause unpleasant side-effects. All patients should be told that if they stop the drug or miss or reduce the dose they may get withdrawal symptoms, usually mild and short-lasting but sometimes severe, especially if the drug is stopped suddenly. Usually treatment is for a relatively brief time, but those who have had two or more episodes in the recent past or whose symptoms have made life particularly difficult may need to continue for as long as two years. There is evidence that St John's Wort might help in mild depression but healthcare professionals have been advised by NICE not to prescribe it because of uncertainty about dose, variety in the strength of different preparations and possible interactions with other drugs, including oral contraceptives, anticoagulants and anticonvulsants. For bipolar disorder, lithium carbonate is the main preventive drug and it is also used for persistent depression that fails to respond to other treatments. All drug treatment for depression needs regular monitoring as the substances have powerful chemical properties with potential side-effects in some people. Furthermore, the nature of the illness means that some sufferers forget or do not want to take the medication.

Some patients with depression – particularly those with manic depression or who are a danger to themselves or to the public, or who are suicidal – may need admission to hospital, or in severe cases to a secure unit, in order to initiate treatment. But as far as possible patients are treated in the community (see MENTAL ILLNESS).

If drug treatments fail, severely depressed patients may be considered for ELECTROCONVULSIVE THERAPY (ECT). This treatment has been used for many years but is now only infrequently recommended, basically to achieve rapid and short-term improvement of severe symptoms after other treatment has failed or if the condition is considered likely to be life-threatening. Given under general anaesthetic, in appropriate circumstances, ECT is safe and effective, though temporary impairment of memory may occur. Because the treatment was often misused in the past, it still carries a reputation that worries patients and relatives; hence careful assessment and counselling are essential and properly informed consent from the patient (or their representative if they are have been deemed to lack CAPACITY to make their own decisions) have to be obtained without pressure or coercion and after a full discussion.