This study evaluates the effectiveness of N-acetyl cysteine (NAC) in the treatment of progressive multiple sclerosis. Half of the patients will receive NAC, while the other half will receive a placebo.

Inclusion Criteria:

• - 40-70 (inclusive) years in age,
• meet 2017 McDonald criteria (Thompson 2018),
• patients with primary or secondary progressive MS (Thompson 2018),
• at least 2 years since progressive symptom onset,
• evidence of clinical changes over the previous 2 years unrelated to relapses: increased EDSS or 20% slowing on 25-foot walk, change of ambulatory support, cognitive change documented on cognitive testing. Progression defined by patients in terms of ambulation perimeter or type of support to ambulate are acceptable if aforementioned physician-based measure changes are not available.
• EDSS score 3.0 to 7.0 (inclusive),
• can be on a stable disease-modifying treatment initiated > 3 months prior to screening,
• can be on stable doses of dalfampridine initiated at least one month before screening.

Exclusion Criteria:

• - MS relapses in the previous 6 months
• oral glucocorticosteroid treatment within the prior 3 months
• patient with issues undergoing MRI scans
• pregnancy or breastfeeding
• women of child-bearing potential not able to utilize an effective form of contraception for the duration of the study
• history of bleeding disorders
• active gastrointestinal ulcers
• abnormal liver function testing (aminotransferase (AST) or alanine aminotransferase (ALT) >2 times upper limit of normal)
• current treatment for active malignancy or metastatic malignancy treated in the past year
• alcohol or substance use disorder
• allergy to NAC
• planned surgery or move within 15 months
• use of medications/supplements with antioxidant properties (including over-the-counter NAC)