Background: The effectiveness of collaborative approaches in health interventions is underlined in the literature. Given the serious challenges to adequately managing the HCV epidemic in people who inject drugs (PWID), and the need to improve existing harm reduction (HR) interventions in this population, it seems important to investigate how collaboration between stakeholders is ensured in action research interventions. The present study aimed to explore interactions between outreach workers and research officers collaborating in the implementation of an action research project for PWID entitled OUTSIDER. Methods: Using three focus groups, we studied the views of 24 outreach workers involved in the implementation and evaluation of a harm reduction educational intervention to help PWID inject more safely in off-site settings. Results: The analysis of participants' discourses highlighted the mixed perceptions they had about OUTSIDER. Several limitations to collaboration emerged. Epistemological (theoretical vs. practical knowledge), methodological (science vs. intervention), axiological (standardised vs. adapted approach), and material (mobilised vs. available resources) issues all placed a burden on the outreach worker-research officer relationship. Outreach workers' acceptance of the project's intervention dimension but rejection of its scientific dimension highlights a lack of contractualisation between the stakeholders involved, and a more general problematisation of the role of outreach workers in implementing action research in HR. How collaboration was perceived and practised by outreach workers participating in OUTSIDER can be considered a reflection of the current challenges to implementing action research in HR. Conclusion: This study of the interaction between the research and implementation dimensions of an action research project explored the tensions between different intervention stakeholders that must work together. Equitable participation and integration of the expertise, practices, and knowledge of all stakeholders involved is essential for successful action research. Given current HCV epidemiological challenges, new forms of cooperation are needed when developing healthcare services and when strengthening collaborative approaches.