The use of the RDSS approach together with peers trained for addiction interviews and urine test made it possible to accurately understand drug use and behavior of the mainly marginalized PWUD population with frequent and poly-drug use.

Our findings contrast with our knowledge before the study. Within 11 weeks, 554 PWUD participated in this study, which confirms that RDSS are highly efficient to recruit participants from this largely marginalized population.

The socio-demographic characteristics of the RDSS participants confirmed a high level of social marginalization and vulnerability of PWUD [11, 19].

Three keys’ findings emerged from this innovative population-based study that were unknown before its implementation. First, heroin/opioid consumption (both self-declared and/or detected in urine) was much higher than previously estimated in studies conducted at addiction care facilities [23]. Our study showed that almost half of the participants reported using heroin at least once in the last month (mainly via sniffing), confirmed by a urine test. Second, only a quarter of our PWUD were enrolled in an addiction care center, and a third of PWID shared their injection equipment with other drug users. Finally, our study confirms the high rate of poly-consumption by drug users and the spread of cocaine and free-base cocaine consumption in France [16].

In 2019, the EMCDDA showed an increase in the use of synthetic opioid and heroin [9]. Both drugs were remaining the most common opioid/opiate on the European drug market although the number of heroin addicts admitted for treatment have declined in most countries in Europe [9]. Brunt et al. [6] collected syringes from automatic injection kit dispensers on streets or at harm-reduction services in seven European cities in 2017 and 2018. The most commonly detected substances were cocaine (31%), heroin (24%), synthetic cathinones (17%), buprenorphine (17%) and amphetamines (17%). In Paris, they found that heroin was detected only in 17% (2017) and 20% (2018) of the analysis of syringes while synthetic cathinones (51% in 2018) and cocaine (49% in 2018) were the most commonly found substances. Using the data collected from addiction care centers in France, the Observatoire Français des Drogues et des Toxicomanies (OFDT) known as the French Monitoring Center for Drugs and Drug Addiction, described a stability in the use of heroin among those aged 18 to 64 years old (1.3%) and a decrease in heroin initiation among those aged 17 years and above from 2014 to 2017 (0.7%), while heroin-related seizures by the police increased [16].

Drug use patterns in France were studied by the ANRS (Coquelicot survey—2011–2013) [23]. This very large survey, involving several French cities, enrolled PWUD from addiction care centers and harm-reduction facilities and used face-to-face interviews conducted by staff specialized in drug use behavior. Our population-based estimates differ from those originating from these addiction/harm-reduction centers in several ways. We noted differences that are firstly explained by the use of peers for interviews, which allowed us to reduce desirability bias and underreporting drug use. Second, the ANRS-Coquelicot study only enrolled PWUD from addiction care centers which may not be representative of the entire PWUD population as those PWUD have probably reduced their drug consumption to improve their health. Finally, the Coquelicot study was not carried out in Montpellier, and we know that wide disparities can exist between cities.

Despite free care for all PWUD at addiction care centers in France, only a quarter of our participants reported having been to such centers over the previous 12 months, and only a third reported having attended a harm-reduction structure. As mentioned before, the strength of RDSS and peer involvement made it possible to reach PWUD who do not typically attend addiction care centers.

The month prior the study, 194 participants reported at least one injection for which injection materials were obtained mainly in harm-reduction centers and/or in pharmacies. However, although injection materials can be obtained free of charge, not all drug users accessed such centers. This is mainly due to barriers such as stigma, misinformation and the absence of motivation. Although NSP and injection assistance have shown to be effective in the reduction of infectious diseases, crime, overdose, and mortality among PWUD [26], our results suggest that the French harm-reduction program has a too limited coverage. This situation may challenge the achievement of HCV elimination in this population. Raising awareness of this situation and developing innovative outreach strategies are urgently needed. For example, educational programs on safer injection practices can reduce syringe sharing and the appearance of cutaneous abscesses [20]. PWID enrolled in these harm-reduction programs receive advice on clean injection techniques and in a few cases, when drug consumption rooms are available, they can inject on-site with sterile materials. However, as for NSP, these educational programs are not sufficiently widespread in France, and in fact, only two drug consumption rooms are available in the whole country. The relevance of such programs in France is currently being studied [2]. Historically, addiction care (CSAPA) and harm-reduction programs (CAARUD) were carried out in different places. We could imagine increasing training and budgets for harm reduction in CSAPAs, in order to systematically offer this notion in the care of PWUD. Finally, the distribution of "Stop addiction" (“Haltes addiction”) structures on the French territory could minimize at-risk behaviors when using drugs.

Finally, our study confirms the effectiveness and efficiency of peers for activities among marginalized populations. Peers involvement has been reported as the most successful method to reach PWUD, screen them for HIV and HCV, engage them in care and provide them with prevention tools [5, 18]. It also plays a key role in harm reduction, positive health-related behavior and proper treatment adherence and compliance [13].

Our study had several strengths. First, a urine test was done to confirm the use of drugs which is not often done in studies in this field. Declarative addictological data are often criticized, but here we see that users declared consumptions that are consistent with what was found in their urine samples. These results are important and will make it possible in the future to interpret with less difficulty the drugs declared by users.

Second, our findings, which include PWUD not enrolled in addiction care or harm-reduction centers, and the use of RDSS have major public health implications as they challenge the current methods of measurement and surveillance of drug use in France and Europe. Furthermore, contrary to popular belief, even if care is free of charge, coverage for medication-assisted treatment is not large. Finally, they also highlight the need to improve current addiction prevention and care programs in France.

In terms of limitations, our study was first a single-site study which represented drug use at a given time and only in a specific city. Despite the interviews being carried out by peers, desirability bias and underestimation of alcohol and PDS use based on self-declaration consumption could not be completely ruled out since financial compensation was an important incentive for participation in the study.

In conclusion, our study highlighted a much higher than expected use of heroin and methamphetamine among PWUD in a large urban area of France. It also raised concern on the poor population coverage of addiction and harm-reduction centers which prompt the need for innovative strategies. As a consequence, equipment sharing stands high among PWUD, questioning the future achievement of HCV elimination in this population. However, further research in other large French cities may be needed to confirm our findings.