Background <p>Vitamin D is widely studied in multiple sclerosis (MS), yet its clinical relevance for disability and relapse activity remains uncertain, particularly in underrepresented populations.</p> Objective <p>To examine whether serum 25-hydroxyvitamin D [25(OH)D] levels are associated with disability or relapse activity in a multicenter cohort of Libyan adults with MS.</p> Methods <p>In a cross-sectional cohort of adults with MS (<i>n</i> = 369), the exposure was 25(OH)D scaled per 10 ng/mL (vitD10). Primary outcomes were the continuous Expanded Disability Status Scale (EDSS) and annualized relapse rate (ARR). We prespecified equivalence margins of ± 0.30 EDSS points and ± 0.25 ARR units, and fitted multivariable models adjusted for demographic, clinical, lifestyle, and treatment-related factors using HC3 robust standard errors. Two one-sided tests (TOST) were used to evaluate equivalence.</p> Results <p>For continuous EDSS, effect estimates were small and the 90% confidence interval lay within the ± 0.30 margin, supporting equivalence. Results for ARR were similarly within the prespecified ± 0.25 margin.There was little evidence of non-linearity, and sensitivity analyses, including measurement-error correction, did not materially change the findings. An exploratory threshold analysis at EDSS ≥ 4 suggested a small increase in odds (OR 1.47, 95% CI 1.01–2.15), which was not supported by continuous analyses and should be interpreted cautiously.</p> Conclusions <p>In this cross-sectional study, differences in 25(OH)D were unlikely to correspond to clinically meaningful differences in disability. These findings suggest the absence of clinically meaningful effects on established disability, while not excluding potential roles in inflammatory activity or earlier disease processes. Prospective studies with longitudinal measurements are needed to clarify temporal relationships and effect modification.</p>