<b><i>Objectives:</i></b> To investigate whether clinical and magnetic resonance imaging (MRI) outcomes of patients with multiple sclerosis (MS) who required a reduction of administration frequency of interferon-beta (IFNB) were similar to those of patients who did not. <b><i>Methods:</i></b> We identified three subgroups of patients under treatment for 24 months with subcutaneous (sc) high-frequency IFNB-1a or -1b: those continuing to receive IFNB according to the drug label (recommended frequency group), those reducing the administration frequency of sc IFNB-1a or -1b (reduced frequency group), and those switched to once weekly intramuscular (im) IFNB (switched group). All patients were followed for further 24 months. The occurrence of relapse, MRI activity and disability worsening were considered as outcome measures. <b><i>Results:</i></b> We identified 308 patients, 201 in the recommended frequency group, 70 in the reduced frequency group, and 37 in the switched group. Patients in the reduced frequency group had increased risk for relapses (HR = 1.95, p < 0.001) and MRI activity (HR = 1.41, p < 0.001), while patients in the switched group had increased risk for relapses (HR = 1.67, p = 0.012), but not for MRI activity (HR = 1.26, p = 0.08) than those in the recommended frequency group. Predictors for disease activity re-start after the reduction of IFNB administration frequency were younger age, higher pre-IFNB relapse rate, and reducing sc IFNB frequency to twice weekly rather than switching to im IFNB-1a once weekly. <b><i>Conclusion:</i></b> Our findings discourage the reduction of sc IFNB administration frequency, especially in younger patients with a higher pre-IFNB relapse rate. However, switching to im IFNB-1a may be considered in some selected cases.