Aim: To investigate whether there is an interconnection between metformin therapy and vitamin B12 deficiency in people with type 2 diabetes and to identify possible risk factors. Study draft: Cross-sectional study. Subjects and methods: 69 patients with type 2 diabetes were included, 28 of them (41 %) who do not have metformin in their therapy (control group) and 41 (59 %) on metformin therapy. Subjects completed two questionnaires, one to assess basic clinical characteristics and the other to assess daily dietary intake of vitamin B12. Also, their serum concentrations of vitamin B12 and homocysteine were determined. Results: The median of serum concentration of vitamin B12 in the control group was 286 pmol/l (interquartile range 239.5 to 369.3), and in the metformin group 252 pmol/l (interquartile range 187.5 to 341), without significant difference between the groups (P = 0.08). In addition, serum concentrations of homocysteine did not differ significantly between the groups (P = 0.67). A total of 20 (29 %) subjects had vitamin B12 deficiency, the percentage is significantly higher amongst patients on metformin therapy (14% vs. 39%, P = 0.03). Sufficient dietary intake of vitamin B12 was present in 20 (41%) subjects with normal serum concentration of vitamin B12 and in 11 (55%) subjects with vitamin B12 deficiency. Risk factors for vitamin B12 deficiency were longer duration of metformin therapy and higher daily and cumulative doses of metformin. Conclusion: Long-term metformin therapy has been associated with vitamin B12 deficiency in persons with type 2 diabetes. Persons on metformin therapy are advised to determine serum concentrations of vitamin B12 in order to begin prompt treatment in the event of a deficiency.