Metformin for Fibromyalgia Symptoms (INFORM Trial)

Inclusion Criteria: * To be able to follow the protocol in English * Fibromyalgia Syndrome: Participant must meet the American College of Rheumatology 2016 revised classification criteria for Fibromyalgia * Ability to take oral medication and be willing to adhere to the metformin regimen (once daily) Exclusion Criteria: * Co-occurring progressive disease (self-report, physician-diagnosed) * Diabetes * Pregnancy or planning to be pregnant in the next year (all premenopausal participants will be tested) * Having known cardiovascular, liver, kidney or pulmonary diseases (self-report, physician-diagnosed) * Having known serious psychopathology (Clinician diagnoses of psychosis, organic mental disorder, or dissociative disorder, self-reported active suicidal intent, self-reported history of inpatient admission to a psychiatric ward in the past year, evidence or self-report of self-injurious behaviors in the past year, reported current or recent history (2years) of non-IV substance abuse, any history of recreational IV drug use) * Having autoimmune disorder (e.g., rheumatoid arthritis) (self-report, physician-diagnosed) * Having neuropathic pain (self-report, physician-diagnosed) * Having pain associated with a terminal illness, acute pain, pain associated with specific organ damage (eg, stomach ulcer) (self-report, physician-diagnosed) * Concurrent use of weight controlling medications (eg, Xenical) * Requiring an interpreter to communicate * Abnormal levels of creatinine, vitamin B12, or hepatic function panel * eGFR of below 45mL/min/1.73m2