Metformin may produce antidepressant effects through improvement of cognitive function among depressed patients with diabetes mellitus.

Diabetes mellitus and depressive disorders are both common chronic diseases that increase functional disability and social burden. Cognitive impairment is a potentially debilitating feature of depression. Previous evidence indicates that the antidiabetic drug metformin could be suitable for diabetic patients with cognitive impairment. However, there is no direct evidence from clinical studies that metformin treatment improves cognitive function in diabetic patients suffering from depression. In the present study, 58 participants diagnosed with depression and type 2 diabetes mellitus (T2DM) were recruited and divided into two groups, one treated with metformin and the other treated with placebo for 24 weeks. Cognitive function, depressive behaviour and diabetes improvement were evaluated. Chronic treatment with metformin for 24 weeks improved cognitive performance, as assessed by the Wechsler Memory Scale-Revised, in depressed patients with T2DM. In addition, metformin significantly improved depressive performance and changed the glucose metabolism in depressed patients with diabetes. Depressive symptoms were negatively correlated with cognitive performance in metformin-treated participants. Furthermore, associations were observed between the parameters of blood glucose metabolism and the depression phenotype. These findings suggest that chronic treatment with metformin has antidepressant behavioural effects and that improved cognitive function is involved in the therapeutic outcome of metformin. The results of the present study also raise the possibility that supplementary administration of antidiabetic medications may enhance the recovery of depression, comorbid with T2DM, through improvements in cognitive performance.