It is widely recognized that injecting drug use is the most harmful route of administration [1, 2]. Compared to non-injecting drug users, injecting drug users are at increased risk for vein damage [3, 4], fatal and non-fatal overdoses [5–7] and the transmission of blood-borne infectious diseases mainly due to sharing injection equipment [8–22].
According to estimates based on 2008 data, there are approximately 17,700 problematic opiate users in the Netherlands . The number of injectors has decreased over the past 15 years due to the high mortality rate and the low rate of initiation into injecting drug use [24, 25]. Compared to other European countries, the Netherlands has (by far) the lowest percentage of injectors : an estimated 7%–13% of the problematic opiate users inject drugs [23, 26].
Although many studies have investigated injecting and non-injecting drug users, few have focused on differences in characteristics between these two groups. Studies comparing injectors and non-injectors have been performed in, for example, North America, Europe and Asia and have reported a variety of differences in characteristics. In terms of sociodemographic characteristics, some studies suggest that non-injectors are more likely to be younger [16, 19, 27] and female . However, other studies found no significant differences between injectors and non-injectors with regard to age and gender [10, 28]. Unstable housing and income from illegal activities are more likely to occur among injectors [10, 19]. Studies also indicate that injectors have a lower education level and are more likely to drop out of school compared to non-injectors [16, 28, 29]. With regard to drug use characteristics, injectors tend to start using drugs at a younger age [27, 28], report a longer duration of drug use [16, 19, 27], a higher frequency of use [10, 27] and higher rates of dependence  compared to non-injectors. Injectors are also more likely to use drugs other than heroin, such as cocaine and/or amphetamine [19, 27]. In terms of health characteristics, injectors more frequently report poor or fair overall health  and report higher levels of somatisation and anxiety symptoms . However, other studies show that non-injectors are more likely to experience mental health problems than injectors .
As mentioned, some studies have aimed to identify differences between injectors and non-injectors. However, no study has investigated the specific situation in the Netherlands. Therefore, little is known about differences in sociodemographic, drug use and health characteristics between injectors and non-injectors in the Netherlands. Furthermore, in the last decade, two notable developments in the Netherlands make a Dutch study of interest. First, intensive efforts from the government, municipality and social care have led to a decrease in the number of homeless problematic drug users [30, 31]. Studies emphasize the potentially positive impact of stable housing on drug-related health, e.g. a decrease in drug use and reduced risk of needle/syringe sharing and unprotected sex [31–34]. Second, in the Netherlands, there seems to be an increasing focus on physical health in low-threshold addiction care, illustrated by the development and use of guidelines, the implementation of HCV screening and the increased number of medical personnel working in these facilities [35–39]. Since nearly all problematic hard-drug users are clients in addiction care, this may have a positive impact on drug-related health. With these developments in mind, the present study aims to investigate key differences between never-injecting drug users (NIDUs), former-injecting drug users (FIDUs) and current-injecting drug users (IDUs) in the Netherlands. More specifically, we examine differences in sociodemographic, drug use and health characteristics between these groups. We also describe the injecting practices of IDUs because of their strong association with health outcomes.