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Abstract
Objective: To evaluate whether generic substitution was associated with any difference in medical resource utilization for 5 widely used antiepileptic drugs (AEDs) in the United States.
Methods: Health insurance claims from PharMetrics Database, representing over 90 health plans between January 2000 and October 2007, were analyzed. Adult patients with epilepsy, continuously treated with carbamazepine, gabapentin, phenytoin, primidone, or zonisamide, were selected. An open-cohort design was used to classify patients into mutually exclusive periods of brand vs generic use of AEDs. Pharmacy and medical utilization were compared between the 2 periods with multivariate regression analyses. Results were stratified into epilepsy-related medical services, and stable (≤2 outpatient visits per year and no emergency room visit) vs unstable epilepsy. Time-to-event analyses were also performed for all services and epilepsy-related endpoints.
Results: A total of 18,125 patients were observed in the stable group and 15,500 patients in the unstable group. After adjustment of covariates, periods of generic AED treatment were associated with increased use of all prescription drugs (incidence rate ratio [IRR] [95% confidence interval (CI)] = 1.13 [1.13–1.14]) and higher epilepsy-related medical utilization rates (hospitalizations: IRR [95% CI] = 1.24 [1.19–1.30]; outpatient visits: IRR [95% CI] = 1.14 [1.13–1.16]; lengths of hospital stays: IRR [95% CI] = 1.29 [1.27–1.32]). Generic-use periods were associated with increased utilization rates in stable and unstable patients and with 20% increased risk of injury, compared to periods with brand use of AEDs.
Conclusions: Generic antiepileptic drug use was associated with significantly greater medical utilization and risk of epilepsy-related medical events, compared to brand use. This relationship was observed even in patients characterized as stable.
AED = antiepileptic drug; CI = confidence interval; ER = emergency room; HR = hazard ratio; ICD = International Classification of Diseases; IRR = incidence rate ratio.
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