The aim of the trial is to demonstrate that 100 mg of oral minocycline twice daily reduces the conversion of CIS to McDonald Criteria MS (McDMS) by an absolute 25% as compared to placebo, over a 6 month follow-up period (primary outcome).

A key secondary outcome is to confirm that this early treatment benefit is maintained at two years.

• Minocycline 100 mg bid orally compared to identical placebo
• Clinically Isolated Syndrome (CIS): Patients with a first clinical demyelinating event suggestive of multiple sclerosis (At baseline participants may meet 2010 McDonald diagnostic criteria for MS, but not 2005 criteria)
• Men and women, aged 18-60y, first event within the previous 180 days; brain magnetic resonance imaging (MRI) with at least two brain T2 lesions which are at least 3 mm in diameter, and at least one of which is ovoid or periventricular or infratentorial.
• Up to 24 months of study drug (Up to 12 months of study drug for patients recruited after December 31, 2012)
• Patients will be discontinued from the study when they convert to McDMS based on the 2005 McDonald definition.
• 12 Canadian MS Clinics
• A total of 154 patients will be randomized. Because 30% of screened patients with CIS who are clinically eligible are not expected to meet the MRI criteria for inclusion, up to 280 patients will be screened.

Inclusion Criteria:

• Age between 18 and 60 years.
• First focal clinical episode suggestive of demyelinating disease within the previous 180 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).
• At least two lesions on the T2-weighted brain MRI* scan at least one of which is ovoid or periventricular or infratentorial. MRI eligibility will be determined centrally by the UBC MS/MRI Research Group.*One lesion on spinal MRI may substitute for one brain lesion as per the 2005 McDonald Criteria.
• 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.

Consent to share IPD was not obtained from subjects so data cannot be shared except amongst investigators.