OBJECTIVES:  To compare outcomes of stable multiple sclerosis (MS) patients on an interferon who stayed on therapy vs. switched to another interferon. METHODS:  MS patients from the MarketScan Commercial administrative claims data who were 18 to 64 years old, relapse-free (stable) over one year while continuously treated with an interferon from January 1, 2010 to March 31, 2015 were identified. Relapses were defined as any MS hospitalizations or any MS outpatient visit within 30 days of an IV steroid use, ACTH, oral steroid daily dose of >= 500 mg prednisone, or total plasma exchange. Patients were propensity-score matched 3:1 using age, gender, previous interferon, adherence, and month and year (first claim=index date) for stable patients who stayed on the initial interferon (No Switch) to stable patients who switched to another interferon (Switch). Patients had to be continuously enrolled for one year prior to the index date (baseline) and one subsequent year after the index date (follow-up). Relapses were recorded for all patients during the follow-up. RESULTS:  After matching, there were 177 patients in the Switch group and 531 in the No Switch group. Baseline characteristics were well matched between groups (47 years old; 79% female). The proportion of patients experiencing a relapse during the follow-up year was significantly higher in the Switch group compared with the No Switch group (13.6%vs. 6.2%, p=0.002). Annual relapse rate was significantly higher in the Switch group compared with the No Switch group (0.15 vs. 0.07, p=0.002). CONCLUSIONS:  MS patients stable on interferon therapy who remain on initial therapy had significantly better outcomes, as measured by less than half the proportion of patients with relapses and half the annual relapse rate, than patients who switched to another interferon. This supports the benefits of allowing patients to remain on current interferon therapy when stable.