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Objectives: The effect of vitamin D supplementation on the disease course of multiple sclerosis (MS) is not established. Neurofilament light chain (NFL) is a sensitive marker of axonal degeneration. The aim of this study was to establish whether high‐dose vitamin D supplementation reduces serum levels of NFL. Materials and Methods: We have performed a 96 weeks placebo‐controlled randomized study of weekly supplementation with 20 000 IU vitamin D3 in 71 patients with relapsing remitting MS (RRMS). Serum levels of NFL were measured at baseline, week 48 and week 96 with a single molecule (Simoa) assay in 69 of these patients. Results: Serum levels of 25‐hydroxyvitamin D more than doubled in the vitamin D group. Compared to placebo, vitamin D supplementation had no overall effect on the change in serum levels of NFL from baseline (P = 0.93 at week 48 and P = 0.56 at week 96). In the subgroup of patients not receiving disease‐modifying therapy, NFL decreased by 30.9% to week 48% and 32.6% to week 96 from baseline in the vitamin D group as compared to the placebo group (P = 0.06 for both time points). Conclusion: With a possible exception for patients not treated with disease‐modifying drugs, weekly supplementation with 20 000 IU vitamin D3 did not affect NFL levels in these RRMS patients. [ABSTRACT FROM AUTHOR]