Background and purposeBeta‐interferons (IFNβ) are the most widely prescribed drugs for patients with multiple sclerosis (MS). However, whether or not treatment with IFNβ can delay secondary progressive MS (SPMS) onset remains unknown. Our aim was to examine the association between IFNβ exposure and SPMS onset in patients with relapsing−remitting MS (RRMS).MethodsA retrospective cohort study using British Columbia (Canada) population‐based clinical and health administrative data (1985–2008) was conducted. RRMS patients treated with IFNβ (n = 794) were compared with untreated contemporary (n = 933) and historical (n = 837) controls. Cohort entry was the first clinic visit during which patients became eligible for IFNβ treatment (baseline). The outcome was time from baseline to SPMS onset. Cox regression models with IFNβ as a time‐dependent exposure were adjusted for sex, and baseline age, disease duration, disability, *socioeconomic status and *comorbidities (*available for the contemporary cohorts only). Additional analyses included propensity score adjustment.ResultsThe median follow‐up for the IFNβ‐treated, untreated contemporary and historical controls were 5.7, 3.7 and 7.3 years, and the proportions of patients reaching SPMS were 9.2%, 11.8% and 32.9%, respectively. After adjustment for confounders, IFNβ exposure was not associated with the risk of reaching SPMS when either the contemporary or the historical untreated cohorts were considered (hazard ratio 1.07; 95% confidence interval 0.93–1.48, and hazard ratio 1.04; 95% confidence interval 0.74–1.46, respectively). Further adjustments and the propensity score yielded results consistent with the main analysis.ConclusionsAmongst patients with RRMS, use of IFNβ was not associated with a delayed onset of SPMS.

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