Despite being a leading cause of death, both in the UK and worldwide, there is little hard evidence to explain why some people attempt suicide.

Most people who choose to end their lives do so for complex reasons. In the UK, research has shown many people who die by suicide have a mental illness, most commonly depression or an alcohol problem.

In many cases, suicide is also linked to feelings of hopelessness and worthlessness.

Vulnerability to suicide

Many experts believe a number of things determine how vulnerable a person is to suicidal thinking and behaviour. These include:

• life history – for example, having a traumatic experience during childhood, a history of sexual or physical abuse, or a history of parental neglect
• mental health – for example, developing a serious mental health condition, such as schizophrenia
• lifestyle – for example, if you misuse drugs or misuse alcohol
• employment – such as poor job security, low levels of job satisfaction or being unemployed 
• relationships – being socially isolated, being a victim of bullying or having few close relationships
• genetics and family history 

In addition, a stressful event may push a person "over the edge", leading to suicidal thinking and behaviour.

It may only take a minor event, such as having an argument with a partner. Or it may take one or more stressful or upsetting events before a person feels suicidal, such as the break-up of a significant relationship, a partner dying or being diagnosed with a terminal illness. 

Mental health conditions

It's estimated 90% of people who attempt or die by suicide have one or more mental health conditions. However, in some cases, the condition may not have been formally diagnosed by a clinician. Conditions leading to the biggest risk of suicide are described below.

Severe depression

Severe depression causes symptoms of low mood, tiredness, loss of interest, despair and hopelessness that interfere with a person's life. People with severe depression are much more likely to attempt suicide than the general population.

Bipolar disorder

Bipolar disorder causes a person's mood to swing from feeling very high and happy to feeling very low and depressed. About one in three people with bipolar disorder will attempt suicide at least once. People with bipolar disorder are 20 times more likely to attempt suicide than the general population.

Schizophrenia

Schizophrenia is a long-term mental health condition that typically causes hallucinations (seeing or hearing things that are not real), delusions (believing in things that are not true) and changes in behaviour. It's estimated that one in 20 people with schizophrenia will take their own life.

People with schizophrenia are most at risk of suicide when their symptoms first begin. This is because they frequently suffer loss at this time – for example, loss of employment and relationships. It's also increased when people with schizophrenia experience depression. The risk tends to reduce over time.

People with schizophrenia are also at increased risk of self-harm.

Borderline personality disorder

Borderline personality disorder is characterised by unstable emotions, disturbed thinking patterns, impulsive behaviour and intense but unstable relationships with other people. People with a borderline personality disorder often have a history of childhood sexual abuse. They have a particularly high risk of suicide.

Self-harm is often a key symptom of this condition.

It's estimated just over half of people with borderline personality disorder will make at least one suicide attempt.

Anorexia nervosa 

Anorexia nervosa is an eating disorder. People with anorexia feel fat and try to keep their weight as low as possible. They do this by strictly controlling and limiting what they eat, as well as sometimes inducing vomiting. It's estimated around one in five people with anorexia will make at least one suicide attempt. Anorexia is associated with a high risk of suicide.

Other risk factors for suicide

Other things that can make a person more vulnerable to suicidal thoughts include:

• being gay, lesbian or transgender, arising from the prejudice these groups often face
• being in debt
• being homeless
• being a war veteran
• being in prison or recently released from prison
• working in an occupation that provides access to potential ways of dying by suicide, such as working as a doctor, nurse, pharmacist, farmer or as a member of the armed forces
• exposure to other people with suicidal behaviour, especially close friends or family members

Antidepressants and suicide risk

Some people experience suicidal thoughts when they first take antidepressants. Young people under 25 seem particularly at risk.

Contact your GP immediately or go to your local hospital if you have thoughts of killing or harming yourself at any time while taking antidepressants.

It may be useful to tell a relative or close friend if you have started taking antidepressants. Ask them to read the leaflet that comes with your medication. Also ask them to tell you if they think your symptoms are getting worse or if they are worried about changes in your behaviour.

Genetics and suicide

Suicide and some mental health problems can run in families. This has led to speculation that certain genes may be associated with suicide.

However, it would be too simple to claim there's a "suicide gene" as the factors leading to suicide are complex and wide ranging. Genetics may influence personality factors (such as acting impulsively or aggressively) that may increase the risk of suicidal behaviour, especially when a person is depressed.

Other theories

An American psychologist called Thomas Joiner developed a theory known as the interpersonal theory of suicide. The theory states three main factors which can cause someone to turn to suicide. They are:

• a perception (usually mistaken) they are alone in the world and no one really cares about them
• a feeling (again, usually mistaken) they are a burden on others and people would be better off if they were dead
• fearlessness towards pain and death

The theory argues fearlessness towards pain and self-harm may be learnt over time, which could explain the strong association between self-harming behaviour and suicide.

People who are regularly exposed to the suffering and pain of others may develop this fearlessness over time. This could help explain why suicide rates are higher in occupations linked to such exposure, such as soldiers, nurses and doctors.

How common is suicide?

During 2012 there were 5,981 suicides in the UK but the number of attempted suicides is much higher.

Suicide occurs in people of all ages, including children, but adults in middle- and late-middle age have the highest suicide rate.