This study examines how professionals within Swedish social services interpret and organise support for adult concerned significant others (CSOs) of individuals with alcohol and other drug (AOD) problems. Previous research has primarily focused on psychosocial interventions, with less attention paid to how CSO support is shaped by professional roles and organisational arrangements. Drawing on interviews with 31 professionals (social workers, AOD therapists and family care counsellors), the analysis applies professional theory, combining Abbott’s system of professions with Brante’s ontological models. The findings show that CSO support is structured through locally negotiated divisions of labour, where professionals occupy peripheral, primary or shared positions. Although professionals draw on different conceptual understandings of CSOs problems, support is consistently oriented toward individualised practices. It is rarely translated into sustained joint CSO–client work and instead managed through professional boundaries and referral practices. The study highlights the emergence of a structural vacuum around relational CSO support. When no profession is organisationally positioned to work with the CSO–client relationship as a unit, relational work becomes everyone’s and no one’s responsibility.