Metformin is associated with decreased mortality and morbidity in stable heart failure patients with diabetes mellitus type II. Diabetic heart failure patients with elevated systolic blood pressure are at increased risk for developing acute decompensated heart failure, which is often associated with decreased kidney function. Metformin-associated lactic acidosis is a rare but fatal side effect that may occur when kidney function is decreased. During acute decompensated heart failure, timely treatment may prevent the decrease in kidney function to the threshold associated with an increased risk of metformin-associated lactic acidosis. Metformin should not be withheld in diabetic patients with stable heart failure who do not have other risk factors for acute decompensated heart failure or lactic acidosis.