Metformin for type 2 diabetes

To the Editor: I enjoyed reading “Should metformin be used in every patient with type 2 diabetes” by Makin and Lansang in the January 2019 issue.¹

I just wanted to point out that metformin is a frequent cause of low serum vitamin B₁₂ levels, and serum vitamin B₁₂ levels should be monitored intermittently in patients using metformin.