OBJECTIVES: To estimate the budget impact of adding delayed-release dimethyl fumarate, a new oral drug indicated for the treatment of relapsing forms of multiple sclerosis (MS) to a managed care formulary in the US.   METHODS: An Excel model was developed to compare the drug-related costs of the current mix of treatments with the costs of an estimated treatment mix including delayed-release dimethyl fumarate for a managed care organization (MCO) with 1,000,000 covered lives. The number of people with relapsing forms of MS was estimated using published prevalence data. Market share of delayed-release dimethyl fumarate was assumed to increase from 10% in 2013 to 25% in 2017 taken proportionately by market shares from all other DMTs. Drug costs included acquisition costs adjusted by patient payments and dispensing fees as well as administration, monitoring and adverse event costs. Annual relapse treatment costs were estimated using the relative risk reduction of a relapse for each DMT derived using a mixed-treatment comparison analysis. A one-way sensitivity analysis was performed. RESULTS: The estimated budget impact of adding delayed-release dimethyl fumarate to the formulary was negative for the first 5 years: in 2014, with a market share of 13.0%, the estimated budget decrease was 0.29% of the total annual costs for DMT-related and relapse treatment costs and a decrease of $0.011 per member per month (PMPM); in 2017, with a market share of 25.0%, the estimated budget decrease was 0.50% of the total annual costs and a decrease of $0.018 PMPM. Sensitivity analyses showed that the model was most sensitive to the acquisition costs of delayed-release dimethyl fumarate. CONCLUSIONS: Under model assumptions for market shares, adding delayed-release dimethyl fumarate to the MCO formulary would result in a small decrease in MCO costs for patients with relapsing forms of MS.