Aims: To investigate and describe the role of drug service user groups in local service user involvement (UI).Method: Mixed-method study involving cross-sectional survey of commissioners, providers and users of drug treatment services and qualitative in-depth case studies in samples of drug action teams in England.Findings: User groups experience problems with sustainability. While the majority of sites reported current or past involvement with user groups, active groups were identified in only a third of sites. These groups varied widely in form and function. Generally groups provided solidarity for users and access to expert opinion for agencies. There was evidence of conflict between user and agency expectations that was exacerbated by power imbalance, role ambiguity and inadequate infrastructure. While the transience of groups was viewed pragmatically by some, others perceived such problems as confirmation that people who use drugs are unable to form effective groups.Conclusions: User groups are fundamental to UI, but are only a partial solution to the challenge of developing UI. Clarity of purpose and process and appropriate support are crucial. Further research should focus on factors that enable groups to function interdependently and contribute to UI without being seen as the exclusive mechanism for it.