What factors impact the likelihood of overdose among people who use drugs?

The number of drug toxicity (overdose) deaths among people who use drugs in Canada is increasing. In 2019, there were 3,721 deaths from opioid overdose in Canada, with 8,480 deaths occurring in 2023.¹ Supervised consumption sites, overdose prevention sites, opioid agonist treatment (OAT), naloxone distribution and safer supply are interventions that mitigate the risk of overdose and other harms associated with drug use. Understanding the individual, social and structural conditions associated with overdose can help identify factors that increase or decrease the risk of overdose, as well as additional strategies for overdose prevention. This article summarizes a systematic review that examined the individual, social and structural factors that impact the likelihood of overdose among people who use drugs.²

What kind of research did the systematic review include?

The review included 26 articles published from 2004 to 2023. An article was included if it:

• included people who use drugs
• examined psychosocial factors, encompassing individual, social and structural conditions that influence a person’s experience and behaviour, and their association with fatal or non-fatal overdose following use of drugs obtained from the unregulated drug supply or illegally obtained regulated drugs

The studies in the review had the following characteristics: 

• Twenty-one studies were conducted in North America, three in Asia, one in Europe and one in Australia.
• Two studies identified overdose through medical documentation. Twenty-four studies identified overdose on the basis of participant reports of a previous non-fatal overdose. Studies had different time periods for including previous overdoses, ranging from the past six months to anytime in the past.
• Twenty-four studies collected quantitative results and used odds ratios, incidence rate ratios, relative risk ratios or hazard ratios to calculate if observed factors had an impact on the likelihood of experiencing an overdose. Two studies collected qualitative results.

Study participants had the following characteristics:

• A total of 150,625 people were included in the 26 studies, with 3,383–4,072, or approximately 3% of these participants, having experienced an overdose. The authors reported a range for the number of participants with a previous overdose as this number was not clearly reported in one study.
• Participant ages ranged from 21 to 56 years.
• In most studies, the majority of participants were male. However, six studies focused on female and/or gender-minority participants.
• Use of multiple substances was reported in 23 studies, including legal substances such as alcohol and illegally obtained pharmaceutical drugs. The remaining three studies did not provide information on the types of substances used.

What factors impact the likelihood of overdose?

The review identified 103 factors associated with overdose and grouped those factors into 10 themes:

• income
• homelessness and housing instability
• incarceration
• traumatic experiences
• overdose risk perception and past experience
• healthcare experiences
• perception of own drug use and injecting skills
• injecting setting
• conditions within physical environment
• social network traits

Income

Eight studies examined the association between income and overdose, including the impact of unemployment or source of income on overdose. Unemployment was associated with increased likelihood of overdose. Receiving social welfare, engaging in sex work, selling drugs and identifying as being in a lower socioeconomic status group also increased the odds of overdose. 

Homelessness and housing instability

Eight studies examined the association between housing and overdose. Homelessness or housing instability was associated with an increased likelihood of overdose. The category of homelessness and housing instability included recent or current homelessness, ever experiencing homelessness or living in a foster home. Unstable housing before incarceration was also associated with an increased risk of overdose after release from incarceration. 

Incarceration

Eight studies examined the association between incarceration and overdose. Past incarceration and recent release from incarceration were associated with an increased likelihood of overdose. Higher odds of overdose were also found among individuals with a history of incarceration and experiences of non-partner violence.

Traumatic experiences

Nine studies examined the association between traumatic experiences and overdose. Physical trauma was found to increase the likelihood of overdose and was the most frequently occurring traumatic experience identified in the review. Physical trauma encompassed physical and sexual violence, including intimate partner violence, with affected populations including women and sex workers.

Other traumatic experiences were also found to increase the likelihood of overdose. Emotional trauma, including experiences of psychological pain and emotional abuse, was found to increase the risk of overdose. Adverse childhood events, such as removal from a family or parental care, increased the odds of overdose, as did caregiver experiences of having a child removed from care or losing custody of a child. Food insecurity also increased the odds of overdose.

Overdose risk perception and past experience

Six studies examined the association between overdose and individuals’ risk perception of and past experiences with overdose. Witnessing an overdose, including overdose of a friend or family member, was associated with increased odds of overdose. Having previously experienced a personal overdose increased the risk of subsequent overdose. Being new to drug use was also associated with a higher risk for overdose. One study found that perceptions of higher severity of past overdoses were associated with decreased overdose incidence, while higher perception of susceptibility to overdose was associated with higher overdose incidence.

Healthcare experiences

Eight studies examined the association between overdose and experiences with healthcare, with studies primarily examining medical treatment for addictions such as opioid agonist treatment (OAT). Experience with addictions treatment was associated with increased likelihood of overdose. Being denied access to addictions treatment and having unmet healthcare needs were also associated with increased likelihood of overdose.

In their discussion of the results, the authors examined possible reasons for the increased likelihood of overdose among people accessing medical treatment for addiction. These reasons included the severity of participants’ substance use disorder and potential suboptimal dosing during OAT, changes in drug tolerance while on OAT and issues related to continuity of care. 

Perception of own drug use and injecting skills

Three studies examined the association between overdose and self-perceived drug use. One quantitative study found increased odds of overdose among participants who considered themselves as having a “drug problem.” The two remaining studies collected qualitative information. Participants who reported a lack of knowledge of drug use, lack of control over the quality of drugs used and lack of knowledge of the effects of those drugs had a higher risk of overdose. One of these studies also found that participants who reported a high degree of knowledge and control over their drug use had an increased risk of overdose.

Two additional studies examined the association between overdose and self-perceived injecting skill. Requiring injection assistance was associated with increased odds of overdose.

Injecting setting

Four studies examined the association between overdose and injecting setting. Injecting in public spaces in the past six months was associated with increased odds of overdose. Injecting alone in the past six months was found to increase the odds of overdose in one study but decrease the odds of overdose in another study. Fear of police while injecting and feeling rushed when injecting outdoors also increased the odds of overdose.

Conditions within physical environment

Six studies examined the association between overdose and conditions within the physical environment. There were conflicting findings on the effect of proximity to harm reduction services such as distribution of safer injecting supplies on odds of overdose. One study found that participants who primarily got their injecting supplies from a harm reduction site had decreased odds of overdose. However, in two studies, participants who accessed safer injecting supplies from a harm reduction service or lived in areas with a high coverage of harm reduction services had increased odds of overdose. 

Living in an area characterized by criminalization, marginalization and drug use were additional physical environment conditions associated with increased odds of overdose. Police-related barriers to accessing harm reduction services were also found to increase the odds of overdose.

Social network traits

Six studies examined the association between an individual’s social network and supports and overdose. One study found that an increasing number of social supports was associated with reduced odds of overdose among women. An increasing density of social networks among people who inject drugs and increasing numbers of people who inject drugs within participants’ networks were found to increase the odds of overdose. Other social network traits that increased the likelihood of overdose included conflict within social networks, being in a friendship or intimate partnership with someone who used drugs, or providing drugs for an intimate partner who was male.

In their discussion of the results, the authors considered the effect that social network composition may have on whether social networks increase or decrease the likelihood of overdose. The authors observed that networks involving conflict, ongoing injecting and exposure to overdose through peers may be associated with harmful impacts to participants and that peer support in other contexts can be beneficial for people who use drugs. 

What are the implications of the review for service providers?

The review identified factors that increased or decreased the likelihood of overdose among people who use drugs. The review focused on psychosocial factors, encompassing individual, social and structural conditions that influence a person’s experience and behaviour. Finding ways to address these factors is an important complement to interventions such as supervised consumption sites, overdose prevention sites, OAT, naloxone distribution and safer supply that mitigate the risk of overdose and other harms associated with drug use.

Key considerations for service providers from this review include the following:

• Factors that contribute to an unstable or unsafe environment for people who use drugs increase their likelihood of overdose. Housing instability, low income or engaging in sex work, and traumatic experiences such as physical or sexual violence consistently increased the likelihood that participants would experience an overdose. Addressing these issues either directly or through referral is a key component of reducing the risk of overdose.
• While increased social supports were found to decrease the likelihood of overdose in some circumstances, social network traits such as conflict within networks or being in a friendship or intimate partnership with someone who used drugs were found to increase the likelihood of overdose. This may be due to multiple factors, including the ability of social networks to support one’s mental and physical well-being but also to potentially create stress on well-being. Programs that provide positive social support for people who use drugs, including support from peers, may help to increase the safety and stability of their environments and decrease the likelihood of overdose.
• Witnessing an overdose, including the overdose of a friend or family member, and having previously experienced a personal overdose increased the likelihood of future overdose. Past overdose experience may add to the traumatic experiences identified in the review, which included violence, emotional trauma and adverse childhood events. Recognizing the current and past trauma of people who use drugs and providing trauma-informed care are important components of reducing the harms they experience.
• Healthcare experiences such as being denied access to addictions treatment and unmet health care needs increased the likelihood of overdose. Education and policy changes that address stigma and barriers to care for people who use drugs are needed to increase their access to healthcare and services to prevent overdose.
• The review identified conditions within the physical environment of people who use drugs that increased the likelihood of overdose. These included police-related barriers to accessing harm reduction services and living in an area characterized by criminalization, marginalization and drug use. Past incarceration also increased the likelihood of overdose. Policy changes that address the criminalization and stigmatization of people who use drugs and that increase their access to services are also needed to address the structural conditions that increase the likelihood of overdose.

When considering this review, it is important to remember the following: 

• The review looked at the association of individual, social and structural factors and overdose, but not the reasons why the associations may exist. Some factors were also found to increase or decrease the likelihood of overdose in different circumstances. In their discussion of the review results, the authors highlighted the interplay between biological, psychological and social conditions that can influence how individual and structural factors impact overdose, which provides some context for why both an increased and decreased likelihood of overdose were observed for some factors. Further research is needed to better understand how factors may impact the likelihood of overdose individually and in combination.
• The varied ways in which each set of study investigators collected and analyzed their data did not make it possible for the authors of the systematic review to combine results in a meta-analysis and determine an overall estimate of how much each factor increased or decreased the likelihood of overdose. Further research that allows us to understand the overall degree to which different factors impact the likelihood of overdose will aid in determining priority areas for service providers addressing overdose among people who use drugs.

What is a systematic review? 

Systematic reviews are important tools for informing evidence-based programming. A systematic review is a critical summary of the available evidence on a specific topic. It uses a rigorous process to identify all the studies related to a specific research question. Relevant studies can then be assessed for quality and their results summarized to identify and present key findings and limitations. If studies within a systematic review contain numerical data, these data can be combined in strategic ways to calculate summary (“pooled”) estimates. Combining data to produce pooled estimates can provide a better overall picture of the topic being studied. The process of pooling estimates from different studies is referred to as a meta-analysis.  

References

1. Federal, provincial, and territorial Special Advisory Committee on Toxic Drug Poisonings. Opioid- and Stimulant-related Harms in Canada. Ottawa: Public Health Agency of Canada; September 2024. Available from: https://health-infobase.canada.ca/substance-related-harms/opioids-stimulants/.
2. Byrne CJ, Sani F, Thain D et al. Psychosocial factors associated with overdose subsequent to illicit drug use: a systematic review and narrative synthesis. Harm Reduction Journal. 2024 Apr 15;21(1):81. Available from: https://harmreductionjournal.biomedcentral.com/articles/10.1186/s12954-024-00999-8.

About the author(s)

Erica Lee is CATIE’s manager of website content and evaluation. Since earning her master of information studies, Erica has worked in the health library field, supporting the information needs of frontline service providers and service users. Before joining CATIE, Erica worked as the Librarian at the AIDS Committee of Toronto (ACT).