Background Recent studies have emphasised the multidimensional nature of the social capital concept, but it is not known whether the health effects of social capital vary by dimension. The objective of this study was to examine the vertical component (ie, respectful and trusting relationships across power differentials at work) and the horizontal component of workplace social capital (trust and reciprocity between employees at the same hierarchical level) as risk factors for subsequent depression.

Methods A cohort of 25 763 Finnish public sector employees who were initially free from depression was followed up on average 3.5 years for new self-reported physician-diagnosed depression and recorded antidepressant prescriptions derived from national registers.

Results Factor analysis confirmed the existence of vertical and horizontal components of workplace social capital. The odds for new physician-diagnosed depression and antidepressant treatment were 30–50% higher for employees with low vertical or horizontal workplace social capital than for their counterparts with high social capital at work. In mutually adjusted models, vertical and horizontal social capital remained independent predictors of physician-diagnosed depression and antidepressant treatment.

Conclusion These results highlight the importance of both vertical and horizontal components of workplace social capital as predictors of employee mental health.