Abstract: We included 22 studies involving 4490 participants. All were randomised trials (3 were cluster RCTs), and 19 of the 22 studies had analysable outcome data. Seventeen of the studies had student populations. Most of the studies were at unclear or high risk of bias for all forms of bias except detection bias. Findings from the five trials with discrimination outcomes (n = 1196) were mixed, with effects showing a reduction, increase or consistent with no evidence of effect. The median standardised mean difference (SMD) for the three trials (n = 394) with continuous outcomes was -0.25, with SMDs ranging from -0.85 (95% confidence interval (CI) -1.39 to -0.31) to -0.17 (95% CI -0.53 to 0.20). Odds ratios (OR) for the two studies (n = 802) with dichotomous discrimination outcomes showed no evidence of effect: results were 1.30 (95% CI 0.53 to 3.19) and 1.19 (95% CI 0.85 to 1.65). The 19 trials (n = 3176) with prejudice outcomes had median SMDs favouring the intervention, at the three following time periods: -0.38 (immediate), -0.38 (1 week to 2 months) and -0.49 (6 to 9 months). SMDs for prejudice outcomes across all studies ranged from -2.94 (95% CI -3.52 to -2.37) to 2.40 (95% CI 0.62 to 4.18). The median SMDs indicate that mass media interventions may have a small to medium effect in decreasing prejudice, and are equivalent to reducing the level of prejudice from that associated with schizophrenia to that associated with major depression. The studies were very heterogeneous, statistically, in their populations, interventions and outcomes, and only two meta-analyses within two subgroups were warranted. Data on secondary outcomes were sparse. Cost data were provided on request for three studies (n = 416), were highly variable, and did not address cost-effectiveness. Two studies (n = 455) contained statements about adverse effects and neither reported finding any. Mass media interventions may reduce prejudice, but there is insufficient evidence to determine their effects on discrimination. Very little is known about costs, adverse effects or other outcomes. Our review found few studies in middle- and low-income countries, or with employers or health professionals as the target group, and none targeted at children or adolescents. The findings are limited by the quality of the evidence, which was low for the primary outcomes for discrimination and prejudice, low for adverse effects and very low for costs. More research is required to establish the effects of mass media interventions on discrimination, to better understand which types of mass media intervention work best, to provide evidence about cost-effectiveness, and to fill evidence gaps about types of mass media not covered in this review. Such research should use robust methods, report data more consistently with reporting guidelines and be less reliant on student populations.

Plain-language summary: We reviewed studies comparing people who saw or heard a mass media intervention about mental health problems with people who had not seen or heard any intervention, or who had seen an intervention which contained nothing about mental ill health or stigma. We aimed to find out what effects mass media interventions may have on reducing stigma towards people with mental health problems. We found 22 studies involving 4490 people.  Five of these studies had data about discrimination and 19 had data about prejudice. We found that mass media interventions may reduce, increase, or have no effect on discrimination. We found that mass media interventions may reduce prejudice. The amount of the reduction can be considered as small to medium, and is similar to reducing the level of prejudice from that associated with schizophrenia to that associated with major depression. The quality of the evidence about discrimination and prejudice was low, so we cannot be very certain about these findings. Only three studies gave any information about financial costs and two about adverse affects, and there were limitations in how they assessed these, so we cannot draw conclusions about these aspects.