Initiation into prescription opioid injection and associated trends in heroin use among people who use illicit drugs

Objectives: Prescription opioid (PO) injection among people who use illicit drugs (PWUD) is an ongoing concern, yet little is known about drug use trajectories associated with initiating PO injection, including potential associations with heroin use. This study aimed to identify predictors of PO injection initiation among PWUD, and examine trends in heroin use before and after initiating PO injection. Methods: Data were merged from three cohorts of PWUD recruited between September 2005 and November 2015. Predictors of PO injection initiation were identified using extended Cox regression models. Trends in heroin use pre- and post-initiation were examined with McNemar's test and compared to matched controls with linear growth curve models. Results: Among 1580 participants, 247 initiated PO injection yielding an incidence density of 3.9 (95% Confidence Interval [CI]: 3.4-4.4) per 100 person-years. In a multivariable analysis, independent predictors of PO injection initiation included heroin injection (Adjusted Hazard Ratio [AHR] = 4.39, 95% CI: 3.24-5.95) and non-injection PO use (AHR = 1.99, 95% CI: 1.25-3.17). In a sub-analysis, compared to matched controls, PO injection corresponded with elevated heroin use post-initiation (p <= 0.05). Discussion: In this study, heroin use and non-injection PO use strongly predicted PO injection initiation. Those who initiated PO injecting had elevated heroin use patterns post-initiation compared to controls. These findings suggest that transitioning to PO injection does not appear to be a substitute for heroin use among PWUD. These findings highlight the importance of addressing PO injection in harm reduction and treatment settings. (C) 2016 Elsevier Ireland Ltd. All rights reserved.