Objective: Highly expensive disease modifying agents (DMAs) were introduced in the 1990s to reduce the frequency of relapse and to slow disease progression in patients with multiple sclerosis (MS). However, the patterns of DMAs use remain largely unknown. This study examines data from 2001-2005 population-based survey of MS patients to estimate duration of DMA use and switching behavior, controlling for patient risk factors. Methods: We examined patterns of DMA use of 670 patients with relapsing remitting (RR) and secondary progressive (SP) MS from the Sonya Slifka Longitudinal Multiple Sclerosis Study. We generated Kaplan Meier covariate-adjusted estimates of survival functions, used Cox proportional hazard models, and adjusted standard errors to account for survey design. Results: The duration of treatment was not different for the 3 interferons and Copaxone, with 72% of patients continuing with the therapy after 3 years, while 1.7% of patients on Novantrone continued treatment after 3 years. Having SPMS increased the risk of terminating DMA by 67% (p=0.01) and having moderate disability increased the risk by 77% (p=0.001) compared with less advanced disease. Older patients and those with longer duration of MS were more likely to stay on DMA. Doctor's advice was the main reason for starting (47%) or stopping (20%) DMA therapy, followed by side effects (10% and 27%, respectively), and burden of drug administration (15% and 10%, respectively). After Avonex, 37% of patients did not take any DMA for at least 6 months, 17% switched to Copaxone and 15% to Rebif. After Betaseron, 51% stayed off DMAs while 19% switched to Copaxone. 37% of Copaxone users switched to no DMA, and 16% switched to Avonex. Conclusion: The majority of MS patients continued treatment for three years and more. Consistent with the risk of cardiotoxicity associated with long-term use, most patients discontinued Novantrone within three years.