Longitudinal associations between loneliness and outcomes of Common Mental Disorders (CMDs): A systematic review and meta-analysis

Abstract Background Loneliness 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. Methods We searched PsycINFO, MEDLINE, Embase and CINAHL from inception to October 2025 for longitudinal studies examining associations between baseline loneliness and CMD outcomes at follow-up. Eligible studies included adults aged 16+ with CMDs. We conducted a narrative synthesis and where appropriate, random-effects meta-analyses for binary depression outcomes reflecting unfavourable course (recurrence, persistence, or non-remission). Study quality was assessed using the Newcastle-Ottawa Scale, and certainty of evidence was evaluated using GRADE. The protocol was registered with PROSPERO (registration number: CRD42023410401). Results We included 22 studies, of which the majority focused on depression-related outcomes (operationalised as symptom severity, recurrence, persistence, remission, and service utilisation). Our narrative synthesis showed consistent associations between higher baseline loneliness and poorer depression-related outcomes at follow-up in people with CMDs across clinical, community, and treatment settings. Meta-analysis of four studies (all in older adults with depression) showed that individuals reporting loneliness at baseline had over three times the odds of poor depression outcomes at follow-up compared with those not lonely (pooled adjusted OR 3.26, 95% CI [2.26–4.70], p < 0.001). This finding should be interpreted with caution, as only one of the four studies adjusted for baseline depressive symptom severity. The few studies capturing suicidal ideation, anxiety, and mixed CMD outcomes found limited and inconsistent evidence. The GRADE certainty of evidence was high for depression outcomes, low for suicidal ideation and anxiety outcomes, and very low for mixed CMD outcomes. Conclusions Loneliness is consistently associated with poorer subsequent depression outcomes in people with CMDs, particularly depressive disorders. These findings suggest loneliness may represent an important prognostic factor in depression and warrant further evaluation in intervention studies. Further high-quality longitudinal research is needed to clarify whether—and under what conditions—loneliness affects outcomes for other CMDs and treatment effectiveness.