Title: Associations between loneliness and outcomes of Common Mental Disorders (CMDs): A systematic review of longitudinal studies

BackgroundLoneliness has been increasingly associated with poor mental health outcomes, yet its prognostic role in people with Common Mental Disorders (CMDs) remains unclear. This systematic review aimed to examine whether loneliness is longitudinally associated with mental health outcomes in individuals with CMDs, regardless of treatment status. MethodsWe searched PsycINFO, MEDLINE, Embase and CINAHL from inception to May 2024 for longitudinal studies examining associations between baseline loneliness and CMD outcomes at follow-up. Eligible studies included adults aged 16+ with CMDs. Study quality was assessed using the Newcastle-Ottawa Scale, and certainty of evidence was evaluated using GRADE. Due to heterogeneity in population and outcomes, a narrative synthesis was conducted. The protocol was registered with PROSPERO (registration number: CRD42023410401). ResultsSeventeen studies met the inclusion criteria. Most included studies investigated depression-related outcomes (n=13), with fewer addressing suicidal ideation (n=5), anxiety (n=2), and mixed CMD outcomes (n=1). Several studies contributed data to more than one outcome category. We found that high baseline loneliness was consistently associated with poorer depression-related outcomes at follow-up in people with CMDs. This association was observed across clinical, community, and treatment settings. In contrast, evidence for suicidal ideation and anxiety outcomes was limited and mixed, with inconsistent associations and lower study quality. Only two studies examined treatment outcomes, with mixed findings on whether loneliness influenced intervention response. Overall, the GRADE certainty of evidence was high for depression, low for suicidal ideation, and very low for anxiety and mixed CMD outcomes. ConclusionsLoneliness appears to be a consistent prognostic factor for poor depression outcomes in people with CMDs, highlighting its potential relevance for assessment and intervention. However, its role in anxiety, suicidality, and treatment response remains unclear. Further high-quality longitudinal research is needed to clarify whether--and under what conditions--loneliness affects broader CMD outcomes and treatment effectiveness.