Heart failure (HF) is a serious and common comorbidity in patients with diabetes mellitus associated with major morbidity and mortality. There is still a large degree of uncertainty related to the most appropriate method for glycemic control in patients with comorbid diabetes mellitus and HF. Historically, metformin has been considered inappropriate in this patient population attributable to concerns over lactic acidosis. Increasing evidence suggests, however, that metformin may be a safe option in patients with HF. In our systematic review evaluating the association between metformin and morbidity and mortality in patients with diabetes mellitus and HF, 9 observational studies were identified. Compared with controls (who were predominantly managed with sulfonylurea therapy), pooled adjusted risk estimates indicated that metformin use was associated with a 20% lower mortality rate. Moreover, no increased risk of mortality was observed in those with reduced left ventricular ejection fraction or chronic kidney disease in those treated with metformin. With respect to overall safety, no increased risk of hospitalization or lactic acidosis was observed in metformin-treated subjects. It is important to consider that all available evidence for the use of metformin in patients with HF comes from observational data. Thus, unmeasured confounding may partially or fully explain our study findings. Until randomized controlled trial evidence is generated, our review supports the current recommendations set by major diabetes mellitus and cardiology clinical practice guidelines, endorsing the use of metformin in patients with diabetes mellitus and HF