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Introduction The metal-chelating activity of metformin, which has long been known but of unclear clinical relevance, has recently been implicated in the pleiotropic effects, including antitumorigenic and anti-inflammatory actions, of the drug. However, whether metformin actually influences metal dynamics in humans has remained unknown. We here investigate whether metformin influences serum metal levels in individuals with type 2 diabetes.Research design and methods In this cross-sectional study, individuals with type 2 diabetes treated or not treated with metformin for at least 6 months were recruited. The primary outcome was the difference in serum copper concentration between metformin users and non-users. Secondary outcomes included differences in serum levels of iron, zinc, and vitamin B 12 as well as in copper-related and iron-related parameters between the two groups.Results A total of 189 individuals (93 metformin users and 96 non-users) were analyzed. Metformin users showed significantly lower serum copper (16.0 vs 17.8 µmol/L, p<0.001) and iron levels (16.3 vs 17.3 µmol/L, p=0.02) and higher zinc levels (13.3 vs 12.5 µmol/L, p=0.01) compared with non-users. Copper-related and iron-related parameters for metformin users were consistent with latent deficiencies of these metals. Serum homocysteine levels (12.2 vs 11.2 µmol/L, p=0.03) were significantly higher, whereas vitamin B 12 levels (338.7 vs 412.8 pmol/L, p<0.001) were significantly lower, in metformin users. Multiple regression analysis including variables that potentially influence metal dynamics identified metformin use as an independent predictor of serum copper (B = –1.54 µmol/L, p<0.001) and iron levels (B = –2.49 µmol/L, p=0.004).Conclusions Metformin use was associated with reduced serum levels of copper and iron, as well as with increased serum zinc levels. These changes in metal dynamics may be related to the pharmacological effects of this widely administered drug.