Background: Metformin for type 2 diabetes mellitus has more beneficial effect with few side effects like vitamin B12 deficiency. So, this study aimed to determine the level of vitamin B12 in patients with type 2 diabetes mellitus using metformin and its association with dose and duration of metformin use. Methods: This was descriptive, cross-sectional, hospital-based study. A total of 128 diabetic patients using metformin for more than 6 months were enrolled. After taking informed consent, considering exclusion criteria, level of serum B12 were quantified by chemiluminescent enzyme immunoassay. Vitamin B12 level <300pg/dl as inadequate and ≥300pg/dl as adequate; metformin 1000mg as low dose; more than 1000mg as high dose were considered. Also, metformin used 1000mg /day taken as constant and comparing it with duration of  > 10 years with duration of > 10 years. Data were entered in Microsoft Excel 2013 and analyzed on SPSS V25. Results: Our study results 62.5% had adequate (>300pmol/l) and 37.5% had inadequate (≤300pmol/l) vitamin B12 level in patients with type 2 diabetes using metformin. Level of vitamin B12 deficiency was statistically associated with increasing dose of metformin used (p<0.001) and long term used of metformin used (p<019). Conclusions: This study concluded that there is prevalence of deficiency of Vit B12 level in type 2 diabetes mellitus patients using metformin. Also, this deficiency level depends on use of dose of metformin.