The prevalence of CIRI among IDU typically ranges from 10% to 30% [7,8]. The variability in prevalence estimates may be due to differences in measurement of the occurrence of CIRI (e.g., reporting current vs. ever having a CIRI), the definition of CIRI (e.g., injection-related vs. any drug-related) and the fact that there are currently no standard guidelines with regard to reporting of CIRI based on severity [4]. Other factors may be the exploration of different risk factors for developing CIRI across settings, for example differences in types of drugs used (e.g., black tar heroin in California, United States vs. white heroin in British Columbia, Canada), intensity of drug use, and availability and access to clean injection paraphernalia [9].

Using longitudinal methods, factors associated with reporting a current CIRI at the univariate and multivariate level are presented in Table ​Table1.1. In univariate analyses, being older and reporting use of the SIF for all injections was associated with a decreased likelihood of developing a current CIRI. Female sex, living in unstable housing, involvement in the sex trade, borrowing a used syringe, requiring help injecting, injecting cocaine daily, injecting heroin daily, and injecting "speedballs" daily were positively associated with reporting a current CIRI.

The observed associations between female sex, daily cocaine injection, living in unstable housing and an elevated risk of having a CIRI are congruent with previous analyses. The link between being female and having a CIRI echoes the findings of previous studies [10,12,13], and may reflect, in part, the complex gender dynamics that exist within injection drug using populations where women are often dependent on men for the attainment and administration of drugs [21].

With regard to the association between cocaine injection and development of CIRI [12,13], cocaine's anaesthetic properties may make it more difficult for individuals to know whether or not they are hitting a vein (as opposed to injecting in the surrounding tissue or skin), resulting in trauma through repetitive attempts to access the vein [22,23]. Missing a vein increases vulnerability for CIRI since injecting into the surrounding tissue creates a niche environment in which bacteria can thrive [9]. Further, due to cocaine's short half-life in comparison to heroin, it is often injected many more times than heroin, which also increases the likelihood of CIRI and transmission of blood-borne viruses such as HIV [16]. Indeed, as indicated by our findings and others, intensity of drug use appears to play a role in CIRI development, as individuals who inject at least once daily have been repeatedly identified to be at elevated risk for developing a CIRI [12,13].

The association between homelessness and an injection site infection has been reported [24]. According to the 'risk environment' framework, as proposed by Rhodes et al., structural and environmental factors are important to consider when assessing risks for drug-related harms as they shape the context in which individual behaviour occurs [24]. It may be that those in our study who reported living in unstable housing may also frequent risky injecting environments, which in turn lead to rushed injections (i.e., not taking time to go through every step of the injection process to ensure a safer injection) or injecting in a high-risk location like the groin for a 'quick fix' [25]. A recent review of homelessness found that between 15–50% of homeless individuals inject drugs, and it was further reported that breaks of the skin were common among such individuals, often leading to bacterial infections due to a lack of hygiene [26]. In addition, the small size, shared facilities and often unhygienic environment of single room occupancy hotels that are common in the DTES promote disease transmission [27].

Among the novel findings in the present study are the associations between CIRI development and borrowing syringes and requiring help injecting. Borrowing used syringes is known to be a strong risk factor for blood-borne viral transmission [28,29]. Our study shows that the transmission of CIRI-related bacteria via sharing of syringes should also be considered by IDU, health professionals, and public health practitioners. However, it is also possible that sharing syringes is not the active vector in this transmission and that this transmission is by other injection drug paraphernalia. Requiring help injecting, a risk factor for CIRI in this study, may increase risk of exposure to bacteria when the individual who is administering the injection injects themselves before injecting the person who requires assistance (i.e., "second on the needle").

This study has several limitations. Firstly, we were unable to examine "skin popping" as an independent variable in this study due to a low number of participants reporting this behaviour. This may be due to the fact that the practice is more commonly associated with injection of "black tar" heroin, a type rarely used in Vancouver. Given that our "skin popping" question in the study questionnaire pertained to intentional "skin popping" it is also possible that participants who injected subcutaneously or intramuscularly by mistake were not captured. Secondly, our study relies on self-report and therefore is potentially vulnerable to social desirability bias. However, we know of no reason to suspect differential reporting between participants with or without CIRI. Thirdly, it is possible that individuals who inject at the SIF are different from those who do not. A study by Wood et al. found that IDU that used the SIF were more likely to be at a higher risk of blood-borne disease infection and overdose compared with IDU who did not use the SIF [30]. Therefore, our results may not be generalizable to the broader local IDU population. However, the SEOSI cohort was randomly recruited from within the SIF [15]. Therefore, we believe that our sample is representative of SIF users. Fourthly, the external validity of this study should be interpreted with caution, as Vancouver's DTES neighbourhood is unique due to its large open drug scene and the high prevalence of cocaine injection. Finally, this study investigates only CIRI related to injection drug use and not other behaviours for example. However, we feel this is an important distinction as it serves to reduce misclassification bias based on reporting CIRI that may be related to other factors such as picking the skin induced by cocaine psychosis [16].