Participatory health research is developing rapidly. Certain issues make the involvement of people with experiential knowledge in research both more complex and more necessary. This is the case for people who use drugs, whose knowledge rendered invisible by the illegality of drug use and the multiple ensuing forms of domination and stigmatization. The purpose of this article is to recount a particular experience of close long-term partnership between a person with experiential knowledge and an academic researcher. Based on a singular collaboration, this article sheds light on the contributions of participatory research, their social and political effects, and their limits. It shows how each stakeholder draws on the knowledge of the other to co-produce research that can help public action evolve and limit epistemic injustices. It also highlights the multiple identities present in this type of collaboration, which foster the conditions for the co-production of knowledge, and suggests ways of enabling similar collaborations with diverse groups.