Because of the state of insulin resistance in heart failure (HF), metformin, with its ability to sensitize tissues to insulin, seems to be an ideal agent for managing type 2 diabetes mellitus (DM) in HF. It reduces the concentration of glucose in blood by enhancing insulin sensitivity, inducing greater peripheral uptake of glucose, and decreasing hepatic glucose output. However, according to the package insert, it is contraindicated in all patients with HF requiring pharmacologic treatment because of increased risk of lactic acidosis. The FDA has now de-escalated this contraindication to a warning as the evidence is lacking regarding an increased risk of lactic acidosis in patients with type 2 DM and HF who take metformin.