The study will consist of two treatment arms: 1) metformin + physical therapy, or 2) placebo + physical therapy. The intervention will consist of a single daily dose of metformin or placebo administered orally for 1 year, in addition to standard-of-care physical rehabilitation. You will complete an MRI, X-Ray, patient reported surveys and an optional blood and urine collection pre-operatively. An optional knee aspiration and tissue biopsy will be conducted in the surgical limb at the time of ACLR. You will complete a questionnaire at 3-, 6-, 9- and 18-months post ACLR. You will also complete follow up assessments at 12- and 24-months including patient reported surveys, X-rays, MRIs and an optional urine/blood collection and physical tests.