Metformin as an add-on or Monotherapy in Treatment of Aging People With Multiple Sclerosis (MS)

The goal of the study is to learn about treating older people with multiple sclerosis (MS) with metformin. Metformin may be used as a single therapy or as an add-on therapy. The investigators want to learn: * The safety and tolerability of metformin extended release (1500 mg/day) as a single therapy or as an add-on therapy in older people with MS compared to placebo * How well metformin protects the nervous system against injury compared with placebo measured by brain MRI over a 9 month treatment period * The effect of metformin to protect brain tissue from age and MS related injury when compared to the placebo group over a 9 month treatment period