Background: Loneliness can occur at any age, but it is more prevalent among older adults due to the associated risk factors. Various interventions exist to improve this situation, but little is known about their long-term effects. Our aims were to determine if these interventions have long-lasting effects and for how long they can be sustained. Additionally, we aimed to analyze if the interventions carried out by volunteers affected the outcomes regarding loneliness and psychological impact. Methods: A systematic review was performed by searching the literature in the MEDLINE PubMed, SCOPUS, Web of Science, PsycINFO, and Web of Science databases for interventions focused on the lonely population. The inclusion criteria for this review were the assessment of loneliness using a validated tool, and loneliness being the primary or secondary outcome. The CASPe checklist was used to assess the risk of bias in the selected studies, and the PRISMA-ScR recommendations were followed to present and synthesize the results. Results: Thirty articles were included. The interventions identified were classified into five categories: psychosocial, technological, health promotion, physical exercise, and multicomponent interventions. Loneliness improved in 24 studies during the post-intervention analysis. Social connectivity and depressive symptoms also improved in most interventions. Long-term follow-ups were conducted with positive results in a total of 16 interventions. Depressive symptoms and social connectivity were also improved. Eight of the interventions were carried out by volunteers and showed good results regarding loneliness. Conclusions: The results obtained in this work suggested that multidisciplinary interventions can reduce loneliness, but more controlled clinical studies are needed.
