Purpose: Metformin is a common medication for patients with hyperglycemia. In adults, one well-documented side effect of metformin is vitamin B12 deficiency. However, this side effect has rarely been studied in pediatric patients. This study examined the changes of vitamin B12 level in pediatric patients being treated with metformin. Methods: Data were collected from pediatric patients (n=151) with at least 3 months of consecutive metformin intake. The effects of dose of metformin on the mean vitamin B12 level were investigated at 6, 12, 24, and 36 months. The effect of compliance of metformin intake on vitamin B12 level also was studied. Results: There was no significant decrease in mean vitamin B12 level at 6, 12, 24, or 36 months in patients treated with metformin. Mean vitamin B12 decrease was only noticeable (p<0.05) in patients taking a high dose of metformin with good compliance. Despite this change, the mean vitamin B12 remained well within the normal reference range. Furthermore, of the 151 patients studied, only 1 demonstrated vitamin B12 deficiency after 12 months of treatment. However, his B12 level was normal at 24 and 36 months without any vitamin B12 supplements. Conclusion Our findings suggest that metformin treatment in children does not cause vitamin B12 deficiency; however, the effect of long-term consistent high-dose treatment on vitamin B12 level remains unknown.