Metformin is associated with low levels of vitamin B12 with no effect on other vitamin levels. A selective action of metformin.

Background and aims: Metformin is frequently used in type 2 diabetes. Long-term treatment is associated with low blood levels of vitamin B12. No studies have evaluated whether metformin affects the blood levels of other vitamins with different mechanisms of intestinal absorption. Thus, the aim of this study was to measure vitamin B12 and other vitamin levels in metformin treated type 2 diabetes patients.

Methods and results: In 200 ambulatory patients with type 2 diabetes, vitamins B12, A, B1, B6, B9, C and E were measured. Subjects were divided into those taking and those not taking metformin. Vitamin levels were compared in these two groups. Metformin significantly reduced the levels of vitamin B12 compared to patients not taking the drug (227.1 ± 96.9 vs 325.6 ± 176.8 pmol/L, p < 0.001), without affecting the levels of all other measured vitamins. A deficiency of vitamin B12 was found in 21.1 % of patients. Metformin tripled the risk of vitamin B12 deficiency in the multivariate logistic regression model.

Conclusions: To the best of our knowledge, this is the first study that shows a specific effect of metformin in reducing the level of vitamin B12 without affecting other vitamin levels.