OBJECTIVE: To estimate the incremental pharmacy PMPM change according to various formulary designs for interferon-beta-1a using administrative claims data METHODS: Cross-sectional sex- and age-specific disease prevalence and treatment rates for multiple sclerosis (MS) patients were measured using integrated medical and pharmacy claims data from a 508,066-member employer group in the southern U.S. Migration to interferon-beta-1a from competitors was based on market-share data for new and existing MS patients. Duration of therapy was estimated by analyzing claims for current MS therapies. Daily therapy cost was provided by the manufacturer, adjusted for migration from other therapies, and multiplied by estimated volume to predict incremental and total per-member, per-month (PMPM) impact. Market-share estimates were used to develop a PMPM forecast for the next two years. PMPM estimates were calculated for preferred and non-preferred formulary tier designs with and without prior authorization (PA). One-way sensitivity analysis was performed to assess influence of product pricing, duration of therapy, and other market factors. RESULTS: Annual incremental PMPM change was $0.047 for the third co-payment tier with PA scenario. The incremental change was greatest for those aged 55 to 65 years ($0.056 PMPM) and did not vary greatly by benefit design. Duration of therapy has the greatest impact on the PMPM estimate across benefit designs. CONCLUSIONS: Interferon-beta-1a will not cause a significant change in managed care pharmacy budgets under a variety of formulary conditions, according to this cross-sectional analysis of current care-seeking behavior by MS patients. Economic impact may differ if interferon-beta-1-a expands MS patients' treatment-seeking behavior.