DPP4 and type 2 diabetes mellitus: Metformin was also not associated with significantly lowered risk of MACE compared to dipeptidyl peptidase-4 inhibitor (DPP-4i) in T2DM patients (HR = 0.95, 95%CI 0.73-1.23; <i>I</i> <sup>2</sup> = 84%).<h4>Conclusions</h4>The effect of metformin on some cardiovascular outcomes was not significantly better than the non-metformin therapy or DPP-4i in T2DM patients based on observational studies.