Interferon beta-1a remains an important treatment option for multiple sclerosis, particularly when safety or tolerability concerns may outweigh the benefits of higher-efficacy disease-modifying therapies. The five-year phase 4 Plegridy Observational Program (POP) study (NCT02230969) collected data on real-world safety and effectiveness of Plegridy® (peginterferon beta-1a) treatment in patients with relapsing multiple sclerosis. To explore the real-world safety and effectiveness of peginterferon beta-1a in patients with relapsing multiple sclerosis, including factors influencing treatment discontinuation. Data were collected prospectively from patients ≥ 18 years old with relapsing multiple sclerosis for overall population analysis and for subpopulations including newly/previously diagnosed patients, age, and experience with peginterferon beta-1a. Outcome measures included annualized relapse rates, adverse events, and predictors of time to treatment discontinuation. Mean (SD) treatment duration in the overall population (N = 1172) was 896.0 (733.15) days. Incidence of adverse events was higher in new than experienced users (79.4% vs. 57.0%). New users were more likely than experienced users to discontinue (hazard ratio = 1.60; P < 0.0001). The adjusted annualized relapse rate was 0.09, and at the end of 5 years, 77.1% of patients were relapse-free. Peginterferon beta-1a is an effective therapy for managing relapsing multiple sclerosis. The identification of predictors of discontinuation can help inform strategies to enhance treatment persistence.