The objectives of this scoping review were to explore the evidence on factors associated with death by suicide among PWUD, to identify gaps in knowledge for future research, and to inform suicide prevention policy and best practice guidelines for working with PWUD, where appropriate. The majority of the evidence reviewed on suicide among PWUD was primary research, originating in Europe, and were based on surveillance systems that captured epidemiological data and trends. Publications generally lacked in-depth analysis of explicit risk factors for death by suicide in PWUD. The most prevalent candidate factor explored was sex, with approximately half of the included publications presenting sex-segregated data for overall numbers of suicide in PWUD. Specific associated factors were commonly not segregated by sex although five studies provided information on the different types of drugs used by men and women who died by suicide [24, 47, 50, 51, 63].

Men account for the majority of completed suicides worldwide [3, 94]. This is thought to be related to male tendency for higher lethality methods of suicide, and the reluctance of men to seek help for depression or suicidal ideation [1, 94]. Freeman et al. [10] argue that suicide attempts in females may represent less of an intention to die, and more a desire to communicate distress or change their social environment. In this review, absolute numbers of death by suicide were higher for men with DUD than for women with DUD, where sex-segregated data were available. However, there was a higher proportion of suicide in women with DUD than men with DUD in several studies [21, 24, 46, 47, 51, 75, 78, 84].

The fact that only five studies reported sex-segregated data highlights a deficit in how studies describing suicide in PWUD report on sex and gender. Where this factor was reported, it was mainly related to biological sex rather than gender identity, resulting in a knowledge gap. A key element to improving the lives of women and girls worldwide is to address sex and gender inequalities, as highlighted in the United Nation’s Global Agenda for Sustainable Development [95]. It is essential to include sex and gender in public health research to bridge the gap in public health knowledge and to advance gender and sex equality.

Increased rates of death by suicide in female PWUD were linked to high rates of mental health problems, high prevalence of opioid and other central nervous depressant drug use, and poor retention in DUD treatment. Lynn et al. [96] identified opioids and antidepressants as the main drugs implicated in suicide drug poisonings among women, a similar finding to the drug profile in females identified in this review. This suggests that female PWUD with depressive disorders are a more vulnerable population than women with depression without a DUD. Although OAT was identified as a protective factor against death by suicide among people who use opioids, poor retention in OAT was associated with increased risk of death by suicide in women. Women with DUD may encounter barriers in accessing OAT in comparison to men, such as stigma, lack of social supports including childcare, and lack of knowledge of services [96–98]. Migrant women may be particularly isolated and unable to access treatment [97], with ethnicity also a candidate factor for suicide in this review.

Mental health conditions increased risk of death by suicide for both men and women who were PWUD, both through the conditions themselves, and through issues concerning dual diagnosis service provision. Previous research has suggested that drug use may exacerbate underlying risk of suicide, or interact with mental illness to increase risk of engaging in suicidal behaviours [8]. Darke and Ross [25] suggest that between a quarter and a third of people who use heroin meet the criteria for a life-time diagnosis of major depression, a figure much higher than levels seen in the general population. Depressive disorders were the most common type of mental health condition reported in publications in the review.

Historically, some mental health services did not accept PWUD for treatment, and some services dedicated to treating SUD may not be equipped to deal with dual diagnosis [90], compounding the mental health difficulties experienced by PWUD with dual diagnosis. Fragmented healthcare services may also make it difficult to recognise when PWUD decrease or change their service use pattern, a risk factor for death by suicide identified by some publications in the review. As PWUD also frequently experience social exclusion [87], also known to be a risk factor for suicide [99], healthcare services may represent an important point of contact. There is therefore a need for integrated addiction and mental health treatment to decrease the risk of suicide in this population [89, 90]. Integrated care models which were shown to improve health outcomes among people with dual diagnosis in clinical trials are complex and challenging to scale-up in real world settings [100]. However, integrated care pathways for people with a dual diagnosis is an important step to ensure provision of services for people with dual diagnosis involves a single care pathway. In Ireland, a new model of care for dual diagnosis, which was developed in partnership with key stakeholders, has recently been launched, which is welcoming [101].

Suicide by hanging has previously been reported as the most common method of suicide in 16 European countries [102]. Darke and Ross [25] identified that people who use heroin were unlikely to use poisoning by heroin as a method of suicide. Violent methods of suicide were more frequent than suicides by poisoning in this review, in publications that reported method of death.

Impulsivity and substance use has previously been associated with suicidal ideation and suicidal behaviour [103]. Stimulant drugs and cannabis were most frequently identified in non-poisoning suicides in the review. Delaveris et al. [60] reported that the non-poisoning suicide illicit drug toxicology profile among PWUD appeared similar to the homicide toxicology profile, whereas the toxicology profile of poisonings was more similar to accidental overdoses. Amphetamines, cannabis, and cocaine were more prevalent in non-poisoning suicide deaths and homicides, while opioids were the most prevalent drug in poisoning suicide deaths and accidental overdoses [60]. Aggression and psychosis prior to suicide was also seen in both men and women who used methamphetamine [57]. This suggests that risk-taking and violent behaviour, including violent methods of suicide, may be increased with the ingestion of stimulant drugs. This differs to the presentation for people who use opioids, with high rates of depression, and therefore may require a different approach to reduce risk of suicide in this population.

Two studies identified race as a risk factor for suicide among PWUD. In people of colour who use drugs, the roles of systemic racism and racial violence may contribute to disproportionate harm, including risk of suicide.

The factors associated with suicide in PWUD identified in this review are those that primarily occur at the level of the individual, although they may point to broader community or societal issues [104]. This was necessary, in order to address the aims of the review, however, we acknowledge that there is a large body of literature exploring the structural conditions in which suicide in PWUD may occur.

Peer-led user movements and advocacy work in the areas of decriminalisation of drugs and expansion of access to harm reduction and social care for PWUD relevant to the local context and needs [105], as well as transformative justice approaches which seek to address the impact of drug-related stigma and harm [106], may also be considered important approaches to suicide prevention [107].