This extensive review delves into the complex relationship between prolonged use of metformin and the possible emergence of vitamin B12 deficiency (VB12D) in diabetic patients. Metformin, a pivotal element in diabetes management, is constantly linked with decreased absorption of vitamin B12, prompting concerns about the enduring consequences of this interaction. The review systematically amalgamates current evidence, elucidating the prevalence, mechanisms, and clinical ramifications of VB12D induced by consistent consumption of metformin. Exploring the different pathways through which metformin might disrupt the absorption of Vitamin B12, the review encompasses interference with the calcium-dependent membrane activity and alterations of the microbiota present in the gut. A meticulous analysis of experimental studies and human trials is undertaken, accentuating the prevalence of variable VB12D among individuals on long-duration treatment of metformin across diverse populations and age groups. Clinical indications of cobalamin deficiency, spanning haematological abnormalities to neurological complications, are systematically examined. Furthermore, the review delves into the potential implications of cobalamin deficiency associated with metformin on diabetes-related complications and overall patient health. This review offers a comprehensive overview of the intricate interplay between the use of metformin and deficiency of vitamin B12 in diabetic patients, emphasizing the importance that lies in routine monitoring, early detection, and personalized interventions to optimize the long-period safety and efficiency of metformin in the treatment of diabetes. It also proposes future research directions to refine clinical guidelines and enhance the understanding regarding the correlation between diabetes, metformin, and vitamin B12.