Metformin use is associated with impaired cognitive performance. Vitamin B12 and calcium supplements may alleviate metformin-induced vitamin B12 deficiency, and might improve cognitive outcomes.
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Moore, EM. Mander, AG. [and] Ames, D. [et al] (2013). Increased risk of cognitive impairment in patients with diabetes is associated with metformin. Diabetes Care, November 2013, Vol.36(10), pp.2981-7. (Click here to view the PubMed abstract).