BACKGROUND Despite extensive evidence regarding the negative effects of stigma experienced by people living with HIV (PLHIV) and people who inject drugs within health care settings, comparatively little evidence exists regarding the effectiveness of initiatives to reduce this stigma.

METHOD This study developed and assessed brief online interventions based on social norms theory with a sample of Australian health care workers (n=653). Participants were randomly allocated to either 1) HIV intervention group, or 2) injecting drug use intervention group. They completed baseline measures of their attitudes towards either PLHIV or people who inject drugs, matching measures of their perceptions of their colleagues' attitudes, plus a series of items reflecting behavioural intentions and agreement with stigmatising behaviour towards PLHIV or people who inject drugs. Participants were presented with a social norms video before completing the measures again.

RESULTS At baseline, participants' agreement with stigmatising behaviour was correlated with their perceptions of how many of their colleagues would agree. After watching the video, participants reported more positive perceptions of their colleagues' attitudes towards PLHIV and people who inject drugs, as well as more positive personal attitudes towards people who inject drugs. Changes in perceptions of colleagues' support for stigmatising behaviour independently predicted changes in participants' personal agreement with that behaviour.

CONCLUSION Findings suggest that interventions based on social norms theory that address health care workers' perceptions of their colleagues' attitudes can play an important role in contributing to broader initiatives to reduce stigma in health care settings.