Trial procedures Patients recruited in the trial were randomized to three groups, metformin 500 mg/d, metformin 1000 mg/d, or placebo. The randomization was conducted by a research assistant, who was blinded to the patient's status. To ensure the concealment of the randomization, the metformin and placebo were provided in coded containers. The appearance of the placebo was identical to that of metformin tablets. All patients, caregivers, and investigators were masked to the randomization.