The interferon-betas are widely prescribed platform therapies for patients with multiple sclerosis (MS). We accessed a cohort of patients with relapsing onset MS from British Columbia, Canada (1995-2013) to examine the potential survival advantage associated with interferon-beta exposure using a marginal structural model. Accounting for potential treatment-confounder feedback between comorbidity, MS disease progression and interferon-beta exposure, we found an association between cumulative interferon-beta use of at least 6 months and improved survival. Sparse follow-up due to variability in patient contact with the health system is one of the biggest challenges in longitudinal analyses. We considered several single-level and multi-level multiple imputation approaches to deal with sparse follow-up of the disease progression information, and both types of approaches produced similar estimates.