OBJECTIVE:To evaluate the safety and efficacy of cholecalciferol in patients with relapsing-remitting MS (RRMS).METHODS:In this double-blind, placebo-controlled parallel-group, 2-year study, 181 patients with RRMS were randomized 1:1. Key inclusion criteria were a low serum 25-hydroxy vitamin D (25OHD) concentration (<75 nmol/L), a treatment with interferon beta-1a 44 μg (SC 3 times per week) 4 months ± 2 months before randomization, and at least one documented relapse during the previous 2 years. Patients received high-dose oral cholecalciferol 100,000 IU or placebo every other week for 96 weeks. Primary outcome measure was the change in the annualized relapse rate (ARR) at 96 weeks. Secondary objectives included safety and tolerability of cholecalciferol and efficacy assessments (ARR, MRI parameters, and Expanded Disability Status Scale [EDSS]).RESULTS:The primary end point was not met. In patients who completed the 2-year follow-up (45 with cholecalciferol and 45 with placebo), all efficacy parameters favored cholecalciferol with an ARR reduction (p = 0.012), less new hypointense T1-weighted lesions (p = 0.025), a lower volume of hypointense T1-weighted lesions (p = 0.031), and a lower progression of EDSS (p = 0.026). The overall rate of adverse events was well balanced between groups.CONCLUSIONS:Although the primary end point was not met, these data suggest a potential treatment effect of cholecalciferol in patients with RRMS already treated with interferon beta-1a and low serum 25OHD concentration. Together with the good safety profile, these data support the exploration of cholecalciferol treatment in such patients with RRMS.