Patterns and correlates of non-fatal heroin overdose at 11-year follow-up: Findings from the Australian Treatment Outcome Study
Introduction
Opioids make the largest contribution to illicit drug-related death, overdose being the leading cause (Bargagli et al., 2006b, Bird, 2010, Darke, 2011, Darke et al., 2011, Degenhardt et al., 2011, Stenbacka et al., 2010). Fatal outcomes, however, represent a minority of overdoses, with 25–50 non-fatal “near misses” for every fatality (Darke et al., 2003). The ubiquity of overdose amongst users is illustrated by lifetime prevalence, with studies consistently reporting a third to two thirds having overdosed (Backmunda et al., 2009, Bargagli et al., 2006b, Britton et al., 2010, Coffin et al., 2007, Darke et al., 2007, Kerr et al., 2007, Wines et al., 2007). Non-fatal overdose is of clinical significance, as it is associated with harms related to the event itself including anoxic brain damage (Warner-Smith et al., 2001, Warner-Smith et al., 2002), subsequent non-fatal overdose (Coffin et al., 2007, Darke et al., 2007) and subsequent death (Darke et al., 2011).
One of the major findings to emerge over the past few decades has been that overdose is not a random event and, hence, is preventable. A number of specific risk factors have been identified. Firstly, as noted above, a history of overdose strongly predicts future overdoses (Coffin et al., 2007, Darke et al., 2007). Heroin overdoses are also associated with more frequent use, higher levels of dependence and with the injection of the drug (Backmunda et al., 2009, Bargagli et al., 2006b, Britton et al., 2010, Coffin et al., 2007, Darke, 2011, Darke et al., 2007, Kerr et al., 2007, Wines et al., 2007). Overdoses, however, rarely involve only heroin, the overwhelming majority involving the concomitant consumption of other CNS depressants (Backmunda et al., 2009, Britton et al., 2010, Coffin et al., 2007, Kerr et al., 2007, Wines et al., 2007, Darke, 2014, Darke et al., 2010). There is also a substantially elevated risk during a relapse to use after a period of abstinence, such as having been in prison, detoxification or naltrexone maintenance (Davoli et al., 2007, Kerr et al., 2007, Wines et al., 2007, Darke et al., 2002, Farrell and Marsden, 2007, Strang et al., 2003). Finally, most overdoses occur when the person is not enrolled in a long-term, stable drug treatment (Davoli et al., 2007, Kerr et al., 2007, Darke et al., 2010).
There are little data on the long-term natural history of opioid overdose, with almost all research to date being cross-sectional. An exception to this has been the Australian Treatment Outcome Study (ATOS) cohort, recruited in 2001–2002, and interviewed at 12, 24 and 36-month follow-up (Teesson et al., 2008). Typical of such cohorts, at baseline 54% reported having a lifetime history of overdose, 22% having overdosed in the preceding year (Darke et al., 2005). Consistent with reductions in reported heroin use, past 12-month overdose declined across follow-up: 12% (12 months), 9% (24 months) and 8% (36 months; Darke et al., 2005, Darke et al., 2007). While there were marked declines in overdose, 20% had overdosed over the course of the first 36 months. By 2009, 31 (5%) of the cohort were deceased, predominately due to overdose (Darke et al., 2011). Indeed, the only significant predictor of mortality was a history of non-fatal overdose.
In this paper, we present new data on the natural history of overdose amongst the ATOS cohort, examining overdose at 11 years, approximately 20 years since the initiation of heroin use. A number of questions arise. Firstly, what are the levels of lifetime overdose exposure approximately 20 years after first heroin use? What is the prevalence of recent (12 month) overdose at 11-year follow-up, and what are the correlates? Specifically, the study aimed to: (1) determine lifetime and recent overdose prevalence amongst the ATOS cohort at 11-year follow-up; and (2) determine correlates of overdose in the year preceding 11-year interview.
Section snippets
Procedure
The data were collected from the New South Wales component of ATOS. Baseline interviews were conducted between February, 2001 and August, 2002. ATOS is a longitudinal study of heroin users, recruited from randomly selected treatment agencies delivering methadone/buprenorphine maintenance treatment (MT) (n = 201), drug free residential rehabilitation (RR) (n = 133) or detoxification (DTX) (n = 201). Subjects were recruited from 19 agencies treating heroin dependence in the greater Sydney region,
Cohort characteristics
The initial cohort consisted of 615 current heroin users, full details of which may be found in Ross et al. (2005). At 11-year follow-up, 431 (70.1%) participants were interviewed, 63 (10.2%) were known to be deceased, 7 (1.1%) were incarcerated and 42 (6.8%) did not wish to participate. A further 21 (3.4%) were confirmed to be alive, but were not interviewed due to repeated cancellations. Overall, we could account for 91.6% of participants. The mean age at 11-year follow-up was 40.0 years (SD
Discussion
To our knowledge, this is the first data to be presented on overdose patterns amongst a cohort some 20 years after heroin initiation. By 11-year follow-up, the proportion who had ever overdosed had risen to over two thirds, with over 2,000 lifetime overdoses reported. While the proportion who had recently overdosed consistently declined across follow-ups, one in 20 had recently overdosed, and one in 10 had done so in the past three years. The correlates of recent overdose, however, remained
Role of funding source
This research was funded by the National Health and Medical Research Council. The NHMRC had no further role in study design; in the collection, analysis and interpretation of data; in the writing of the report; or in the decision to submit the paper for publication.
Contributors
Professors Darke conducted the statistical analyses and write-up of the paper. Professors Teesson and Darke, Associate Professors Mills, Slade; Burns and Dr Ross were Chief Investigators on the project. Dr Marel was the project coordinator, involved in data collection and entry. All authors contributed to and have approved the final manuscript.
Conflict of interest
None to declare.
Acknowledgements
This research was funded by a National Health and Medical Research Council project (grant number 1009934), and NHMRC Fellowships to Maree Teesson and Katherine Mills. The National Drug and Alcohol Research Centre at the University of NSW is supported by funding from the Australian Government. The authors wish to thank Joanne White, Sonja Memedovic, Sarah Ellis, Philippa Ewer, Louise Mewton, the original ATOS research team and all participating agencies.
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