Objectives: To study vitamin B12 concentrations in patients with type 2 diabetes with and without metformin use and to identify risk factors and consequences of low vitamin B12 concentrations. Research design and methods: This study had a cross-sectional design. During eight weeks all patients with type 2 diabetes visiting the diabetic outpatient clinic of the Isala Clinics in Zwolle were approached for participation. Participation included measurement of haemoglobin, mean corpuscular volume and vitamin B12 levels. Data on neuropathy were retrospectively searched for in the patient records. Vitamin B12 deficiency was defined as serum B12 concentrations <150 pmol/l. Results: In the total cohort (n=298), the overall prevalence of vitamin B12 concentrations <150 pml/l was 9.7% (95% CI
6.6-13.7%). In type 2 diabetes patients not taking metformin (n=134), the prevalence was 4.4% (95% CI 1.6-9.4%) compared with 14.1% in metformin users (n=164) (95% CI 9.2-20.4%; p=0.006). Each 100 mg step in metformin dose increased (OR=1.081, p=0.014), whereas PPI use lowered (OR=0.322, p=0.037) the odds of having a vitamin B12 deficiency in logistic regression. Nevertheless, metformin use did not predict the chance on having anaemia or neuropathy. Conclusion: Among patients with type 2 diabetes using metformin, the prevalence of vitamin B12 deficiency is higher than compared with patients not using metformin. However, metformin use did not predict the chance of having anaemia or neuropathy.