Peer support workers are now working with patients in a variety of settings, coming into close contact and even work alongside health professionals. Despite the potentially influential position peer support workers hold in relation to those engaged in support activities, their role, duties and their relationship to peers and health professionals lack clarity and is often defined by other actors. This study explores how peer support workers interpret and define the activities, responsibilities and knowledge associated with their work. Using methods of membership categorisation analysis, we analysed interview materials generated by conducting individual semi-structured interviews during the autumn of 2016 with prostate cancer peer support workers ( n = 11) who currently volunteer as support workers in Finland. Although the peer support workers acknowledged the psychosocial aspects of the work, we argue that their interpretations extend far beyond this and encompass expertise, advocacy and activism as central features of their work. These can be used to strengthen their position as credible commentators and educators on issues relating to cancer and men's health; raise awareness and represent the 'patient's voice' and attempt to influence both policy and clinical practice. These findings suggest that by categorising their work activities in different ways, voluntary sector actors such as peer support workers can attempt to portray themselves as legitimate authorities on a range of issues and influence decision-making ranging from individual level treatment decisions all the way to health policy.