The purpose of this study is to determine if teriflunomide will be safe and effective to prevent relapses in patients with relapsing types of MS when switching from natalizumab to teriflunomide in patients at risk for PML. This is a two center interventional study of patients who have had 12 or more continuous infusions of natalizumab , who are anti-JCV-ab positive, and who had been free of clinical relapses during prior 12 months of natalizumab therapy who will be switching to teriflunomide.