Elsevier

Drug and Alcohol Dependence

Volume 162, 1 May 2016, Pages 51-55
Drug and Alcohol Dependence

Full length article
Non-fatal overdose as a risk factor for subsequent fatal overdose among people who inject drugs

https://doi.org/10.1016/j.drugalcdep.2016.02.024Get rights and content

Highlights

  • We assessed risk factors for overdose death among two prospective cohorts of persons who inject drugs (PWID).

  • Recent non-fatal overdose was independently associated with subsequent overdose mortality.

  • A dose-dependent relationship was found between fatal overdose and number of previous overdoses.

Abstract

Objectives

To examine the relationship between non-fatal overdose and risk of subsequent fatal overdose.

Methods

We assessed risk factors for overdose death among two prospective cohorts of persons who inject drugs (PWID) in Vancouver, Canada. Extended Cox regression was used to examine if reports of non-fatal overdose were associated with the time to fatal overdose while adjusting for other behavioral, social and structural confounders.

Results

Between May, 1996 and December, 2011, 2317 individuals were followed for a median of 60.8 months. In total, 134 fatal overdose deaths were identified for an incidence density of 8.94 (95% confidence interval [CI]: 7.55–10.59) deaths per 1000 person-years. During the study period there were 1795 reports of non-fatal overdose. In a multivariate model, recent non-fatal overdose was independently associated with the time to overdose mortality (adjusted hazard ratio [AHR] = 1.95; 95% CI: 1.17–3.27). As well, there was a dose response effect of increasing cumulative reports of non-fatal overdose on subsequent fatal overdose.

Conclusion

Reports of recent non-fatal overdose were independently associated with subsequent overdose mortality in a dose-response relationship. These findings suggest that individuals reporting recent non-fatal overdose should be engaged with intensive overdose prevention interventions.

Introduction

Globally, the nonmedical use of drugs represents a considerable public health burden and the human and financial costs have been a growing focus of research over the past decade. Of all the harms associated with illicit drug use, among the most direct and important consequences is fatal overdose. In the United States, for instance, accidental overdose has recently emerged as a leading cause of death (Mack, 2013, Murphy et al., 2013). Similarly, overdose mortality has been a longstanding concern, particularly among persons who inject drugs (PWID; Mathers et al., 2013, Quan et al., 2011).

In this context, considerable research has been dedicated to identifying risk factors for overdose mortality with many past studies examining toxicology reports and other circumstances of overdose death (Preti et al., 2002, Zamparutti et al., 2011). Similarly, significant energy has gone into identifying risk factors associated with non-fatal overdose (Darke et al., 2005, Mathers et al., 2013). Substantially more common than fatal overdose (Warner-Smith et al., 2002), non-fatal overdoses are the cause of significant morbidity with a proportion of individuals suffering from hypoxia, aspiration or other negative health outcomes (Britton et al., 2010, Warner-Smith et al., 2002, Warner-Smith et al., 2001). The reported prevalence of non-fatal overdoses among illicit drug users has varied significantly based on geography and study population, with estimates ranging between 20% and 70% per lifetime (Darke et al., 1996, Kerr et al., 2007, Kinner et al., 2012, Ochoa et al., 2001, Silva et al., 2013).

One important question, which remains insufficiently addressed in the literature, is to what degree non-fatal overdose events are associated with the risk of subsequent fatal overdose. It may be that persons who experience non-fatal overdose may become more cautious in their future drug use thereby reducing the risk of future adverse events (Mathers et al., 2013), or that those experiencing non-fatal overdose are at higher risk of subsequent fatal overdose (Coffin et al., 2007, Stoové et al., 2009). Due to the urgent public health crisis surrounding fatal overdose mortality, the present study was undertaken to examine if self-reported non-fatal overdose was associated with subsequent fatal overdose among PWID in a Canadian setting.

Section snippets

Methods

Data for the present study were derived from the Vancouver Injection Drug Users Study (VIDUS) and AIDS Care Cohort to Evaluate Access to Survival Services (ACCESS), which are open prospective cohorts of persons who use drugs in Vancouver, Canada. The studies have been described in detail previously and are essentially identical in their recruitment, data collection and follow up procedures with the exception of HIV status and an eligibility criterion regarding injection drug use (Kerr et al.,

Results

In total, 2598 participants were recruited and followed between May 1996 and December 2011. Overall, 281 (10.8%) individuals were excluded as a result of missing follow-up information or incomplete data. Those participants who were excluded were younger (p < 0.05), however, there was no difference in baseline reports of recent non-fatal overdose or any of the other variables considered.

For the 2317 participants eligible for the present study, the median follow up time was 60.8 months

Discussion

In the present study, we observed an elevated risk of death from overdose in individuals who had recently reported non-fatal overdose. In addition, a dose-response effect was observed with an increasing number of cumulative reports of non-fatal overdose associated with a greater risk of subsequent overdose death. This finding has significant implications for future research and public health practice.

Our study is among the few to investigate the relationship between non-fatal overdose events

Funding sources

This study was supported through the US National Institutes of Health through grants that support the VIDUS (U01DA038886) and ACCESS studies (R01DA021525). This research was undertaken, in part, thanks to funding from the Canada Research Chairs program through a Tier 1 Canada Research Chair in Inner City Medicine which supports Dr. Evan Wood. Dr. Kanna Hayashi is supported by the Canadian Institutes of Health Research New Investigator Award (MSH-141971). Dr. M-J Milloy is supported in part by

Conflict of interest

None.

Contributions

EW and TK contributed to the study design. AC drafted the manuscript. KH, MM and EW reviewed and edited draft and contributed to development of discussion. HD performed statistical analyses. All authors edited and have approved the final article

Acknowledgments

The authors thank the study participants for their contribution to the research, as well as current and past researchers and staff. We thank Deborah Graham, Peter Vann, Tricia Collingham, Cody Callon, Jennifer Matthews, Guillaume Colley, Sabina Dobrer, Caitlin Johnston, Jenny Matthews, and Steve Kain for their research and administrative assistance.

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