Interventions promoting employee “speaking-up” within healthcare workplaces: A systematic narrative review of the international literature

•Implementation of speak-up interventions are either effective, ineffective or indeterminate.•The development of speak-up interventions rarely consider and build-upon existing research findings.•Local speak-up interventions rarely consider and build-upon national or trans-national policy directives.•Speaking-up is shaped by socio-cultural norms and values existing out-with the organisation.