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ABSTRACTObjectives:To explore the effects of generic substitution of the antiepileptic drug (AED) topiramate (Topamax†) in Canada; to convert observed Canadian costs into the settings of France, Germany, Italy, and the United Kingdom (UK); and to forecast the economic impact of generic topiramate entry in these four European countries. †Topamax is a registered trade name of Ortho-McNeil Neurologics, Inc., Titusville, NJ, USAResearch design and methods:Health claims from Régie de l''assurance maladie du Québec(RAMQ) plan (1/2006–9/2008) and IMS Health data (1998–2008) were used. Patients with epilepsy and ≥2 topiramate dispensings were selected. An open-cohort design was used to classify observation into mutually-exclusive periods of branded versus generic use of topiramate. Canadian healthcare utilization and costs (2007 CAN/person-year) were compared between periods using multivariate models. Annualized per-patient costs (2007€ or 2007£/person-year) were converted using Canadian utilization rates, European prices and service-use ratios. Non-parametric bootstrap served to assess statistical significance of cost differences. Topiramate market was forecasted following generic entry (09/2009–09/2010) using autoregressive models based on the European experience. The economic impact of generic topiramate entry was estimated for each country.Results:A total of 1164 patients (mean age: 39.8 years, 61.7 female) were observed for 2.6 years on average. After covariates adjustment, generic-use periods were associated with increased pharmacy dispensings (other AEDs: 0.95/person-year, non-AEDs: 12.28/person-year, p < 0.001), hospitalizations (  0.08/person-year, p  0.015), and lengths of hospital stays (0.51 days/person-year, p < 0.001). Adjusted costs, excluding topiramate, were CAN1060/person-year higher during generic use (p  0.005). Converted per-patient costs excluding topiramate were significantly higher for generic relative to brand periods in all European countries (adjusted cost differences per person-year: €706–815, p < 0.001 for all comparisons). System-wide costs would increase from 3.5 to 24.4 one year after generic entry.Limitations:Study limitations include the absence of indirect costs, possible claim inaccuracies, and IMS data limitations.Conclusions:Higher health costs were projected for G4 European countries from the Canadian experience following the generic entry of topiramate. [ABSTRACT FROM AUTHOR]