The aim of this integrative literature review was to evaluate the efficacy and safety of Metformin as an add-on therapy to insulin in poorly controlled overweight adolescents with type 1 diabetes mellitus. The research problem centered on providing optimum disease management during the most critical growth period and reducing the potential for cardiovascular-related morbidity and mortality in the future. The findings suggested that Metformin, in conjunction with insulin therapy, helped patients to achieve better metabolic control. The quality of metabolic control varied between studies according to differences in study design, exclusion and inclusion criteria, and methods. Adjunctive Metformin therapy has a positive effect on diabetes management, treatment, and prevention of cardiovascular-related complications with a minimal risk of side effects. Suggestions for further exploration of the research results and clinical implications are included in the review.