The safety of dipeptidyl peptidase-4 (DPP-4) inhibitors or sodium-glucose cotransporter 2 (SGLT-2) inhibitors added to metformin background therapy in patients with type 2 diabetes mellitus: a systematic review and meta-analysis - Kawalec, P; Mikrut, A; Lopuch, S (Diabetes Metab Res Rev, 2014)