Inclusion Criteria:

• Age between 18 and 50 years.
• First focal clinical episode suggestive of demyelinating disease within the previous 90 days (measured from onset of the first symptom to treatment start), based on the appearance of a neurological abnormality, present for at least 24 hours. Objective clinical evidence must be present or documented. Patients will be included irrespective of whether the first clinical demyelinating episode was monosymptomatic (i.e. clinical evidence of a single lesion) or polysymptomatic (i.e. clinical evidence of more than one lesion). The time between the first clinical event and initiation of treatment is reasonably short to prevent loss of patients that convert to MS early. While previous CIS studies required earlier enrolment but these studies found that few (about 5 %) patients have a second relapse within 30-60 days of enrolment so we do not expect this extension of the enrolment period to introduce significant bias. On the other hand a 90 day enrolment period increases the generalizability of the results and will improve recruitment.
• At least two lesions on the T2-weighted brain MRI scan with a size of at least 3 mm, at least one of which is ovoid or periventricular or infratentorial. MRI eligibility will be determined centrally by the UBC MS/MRI Research Group.
• Sexually active women of child-bearing potential must agree to use adequate contraception.
• Written informed consent

Exclusion Criteria:

• Any disease other than MS that could better explain the patient's signs and symptoms.
• Any previous clinical event reasonably attributable to acute demyelination, regardless of whether medical attention was obtained.
• Complete transverse myelitis or bilateral optic neuritis. A waiver can be obtained for bilateral optic neuritis but must be obtained prior to randomization. Waivers must be approved by 3 neurologists including a member of the Clinical Eligibility / Endpoint Committee, a member of the DSMC, and by an experienced MS neurophthalmologist.