Long-term metformin therapy is associated with reduced vitamin B12 status, but the clinical significance of biochemical abnor malities and optimal monitoring remain uncertain. This narrative review evaluates evidence on mechanisms, risk factors, clinical consequences, diagnosis, monitoring, and management, with emphasis on clinically actionable risk stratification and recent guidance. Targeted, non-systematic PubMed/MEDLINE and reference-list searches identified relevant English-language evidence primar ily from 2000 to August 2026. Earlier seminal studies were retained when relevant. Randomised, longitudinal, observational, and evidence-synthesis studies and current ADA, NICE, and MHRA guidance were prioritised. No preregistered protocol, formal inclusion/exclusion criteria, or risk-of-bias assessment was applied. Long-term metformin exposure is consistently associated with lower circulating vitamin B12 concentrations and biochemical defi ciency; higher dose and longer or cumulative exposure increase risk in several studies. Low serum vitamin B12 concentrations are not equivalent to clinical vitamin B12 deficiency. Neurological manifestations may occur without anaemia and overlap with diabetic neuropathy. Total or active vitamin B12 are appropriate initial tests, while methylmalonic acid may clarify indeterminate results. Cur rent guidance favours symptom-triggered testing and periodic monitoring in higher-risk users over universal screening. Current guidance supports symptom-triggered testing and periodic monitoring in higher-risk patients rather than universal screening. Metformin can generally be continued while confirmed deficiency is treated. Oral or intramuscular vitamin B12 re placement should be individualised according to deficiency severity, malabsorption, neurological involvement, adherence, and patient preference. Further prospective studies are needed to define optimal monitoring intervals and the role of preventive supplementation.