Managing Chronic Tendon Pain by Metformin

Inclusion Criteria: * Willing and able to provide informed consent for participation * Clinical diagnosis of Achilles tendinopathy * Pain disability index score of between 20 - 70 points (0-100 scale VISA-A) * Sonopalpation tenderness with tendon thickening and/or loss of fibrillary architecture. * Ability to read, speak, and understand English Exclusion Criteria: * Any intervention (corticosteroids or other orthobiologic intervention as injection) to targeted tissue within the last 6 months. * Previous Achilles tendon surgery. * Known platelet abnormality or hematological disorder. * Presence of other conditions that might affect the functional scale of the indexed limb. * Unwillingness to be randomized. * Body Mass Index (BMI) greater than 30. * Occult tear in the indexed tendon. * Systemic inflammatory disease. * Use of fluoroquinolone in the past 6 months. * Subjects must not be pregnant or breastfeeding, or planning to become pregnant or breastfeed during the course of the trial * Chronic kidney disease (eGFR \<60) * Diagnosis of diabetes mellitus or elevated HbA1c at screening (\> 6.5) * Unable to take an oral medication in a non crushable pill form * Taking metformin presently or within the last 6 months * History of allergy to metformin * History of lactic acidosis or elevated lactate at screening (\> 2.2) * Severe Hepatic dysfunction * Currently taking a diabetic medication such as sitagliptin, saxagliptin, linagliptin, alogliptin, sitagliptin with metformin * Currently taking a Carbonic anhydrase inhibitor such as topiramate, zonisamide, acetazolamide, dichlorphenamide, methazolamide * Currently taking cimetidine