The primary end point is the number of migraine days per month during treatment with metformin vs placebo. The secondary end point is the percentage of patients with >50% reduction in migraine days per month (response rate). Adverse events will be recorded. Mean 4-week migraine frequency for patients receiving metformin vs placebo treatment will be assessed by using the Hills-Armitage method. Response rate will be assessed by using the McNemar test. The statistician will create Web-based case report forms using REDCap. Study coordinators will record data using REDCap forms.