Assuring participant confidentiality in illicit drugs research has raised legal questions and challenges both for researchers and ethics committees. There are similar challenges for clinicians. To study cannabis use in Aboriginal people in Arnhem Land (Northern Territory), a risk-management approach was successful. Aboriginal participants were informed in their own language that confidentiality could not be assured if they disclosed information about illegal behaviours. Researchers avoided questions of intrinsic interest to law enforcement. Relationships between researchers and study participants and the integrity of the study were preserved. These considerations have relevance for clinicians as well as researchers dealing with the influence of illicit behaviours on health.