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