OBJECTIVES: Multiple sclerosis (MS) is a chronic incurable disease with a progressive course.  Studies have demonstrated that disease progression can be slowed by treatment with disease modifying therapies (DMTs). Poor DMT adherence is associated with a higher risk of relapse, lower quality of life and higher medical costs. The adherence measure, persistency, refers to the duration a patient continues with therapy.  This study aims to evaluate the non-pharmacy medical costs associated with the persistency of DMT’s in patients with MS.  METHODS: A decision-analytic model was designed using persistency rates, health care resource use and costs from the published literature.  The model evaluated the medical costs associated with the persistence, switching, and discontinuation rates of patients on interferon β1a IM, interferon β1b SC, and interferon β1a SC over a 2-year time horizon. Using 6-month intervals in the model, patients could switch to a comparator treatment, discontinue treatment, or persist/remain on their current treatment, and would incur the non-pharmacy medical costs associated with each. For patients who switched treatments, the model assumed an equal probability of switching to the three remaining DMTs.  The model also assumed that patients who discontinued therapy would initiate on a different therapy after one month of discontinuing. The overall medical costs related to persisting on treatment, switching treatment or discontinuing treatment were calculated for all treatment arms. RESULTS: Based on the results from the model, an annual per-patient medical cost of $7423 was observed for interferon β1a IM patients, showing a medical cost advantage over patients on interferon β1b SC ($8144; 8.9%), and interferon β1a SC ($7552; 1.7%). CONCLUSIONS: MS is a lifelong disease that requires continuous treatment.  The results of this model show that interferon β1a IM is a cost-saving treatment for the medical costs associated with MS patients on disease modifying therapies.