AbstractBackground and purposeThis study assessed the effect of patient characteristics on the response to disease‐modifying therapy (DMT) in multiple sclerosis (MS).MethodsWe extracted data from 61,810 patients from 135 centers across 35 countries from the MSBase registry. The selection criteria were: clinically isolated syndrome or definite MS, follow‐up ≥ 1 year, and Expanded Disability Status Scale (EDSS) score ≥ 3, with ≥1 score recorded per year. Marginal structural models with interaction terms were used to compare the hazards of 12‐month confirmed worsening and improvement of disability, and the incidence of relapses between treated and untreated patients stratified by their characteristics.ResultsAmong 24,344 patients with relapsing MS, those on DMTs experienced 48% reduction in relapse incidence (hazard ratio [HR] = 0.52, 95% confidence interval [CI] = 0.45–0.60), 46% lower risk of disability worsening (HR = 0.54, 95% CI = 0.41–0.71), and 32% greater chance of disability improvement (HR = 1.32, 95% CI = 1.09–1.59). The effect of DMTs on EDSS worsening and improvement and the risk of relapses was attenuated with more severe disability. The magnitude of the effect of DMT on suppressing relapses declined with higher prior relapse rate and prior cerebral magnetic resonance imaging activity. We did not find any evidence for the effect of age on the effectiveness of DMT. After inclusion of 1985 participants with progressive MS, the effect of DMT on disability mostly depended on MS phenotype, whereas its effect on relapses was driven mainly by prior relapse activity.ConclusionsDMT is generally most effective among patients with lower disability and in relapsing MS phenotypes. There is no evidence of attenuation of the effect of DMT with age.