This will be a single-center, double-blind, randomized, placebo-controlled, crossover study. The investigators will enroll 40 subjects. Subjects will be recruited from the outpatient Mayo Headache Clinic, Mayo Women's Health Clinic, Mayo Community Internal Medicine, and the Mayo Clinic- Arizona State University Neurology Clinic. The Headache Clinic employs 6 headache specialists and 1 fellow which is sufficient staffing to support this study and recruitment of patients with migraine.~Before study enrollment, patients will undergo a 2-hour oral glucose tolerance test to confirm the absence of diabetes mellitus.~Subjects will then keep a headache diary for 4 weeks to establish baseline characteristics, including number of migraine days per month, number of moderate to severe headache days per month, and number of days per month of acute medication use. The 4-week baseline period will be followed by 2 treatment periods of 12 weeks each, separated by a washout period of 4 weeks (total study duration, 32 weeks). For the 12-week treatment periods, subjects will receive either metformin 500mg twice daily or matching placebo twice daily.~Subjects will have 7 study visits. Routine laboratory studies and a pregnancy test will be performed at enrollment and week 20. Abnormal kidney or liver function will exclude subjects. Pregnancy will exclude subjects. General and neurologic examinations will be performed during 3 of the 7 visits. At each visit, subjects will receive medication for the period until the next visit. A headache diary will be used for the trial. For every day that the subject has a headache of any type, she or he will record headache characteristics, associated features, and medications used.
The investigators will evaluate the safety and efficacy of metformin for preventive treatment of migraine in a randomized, double-blind, placebo-controlled, crossover trial.