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MICHAEL DICKSON, PH.D., is Professor, College of Pharmacy, University of South Carolina, Columbia. THOMAS J. BRAMLEY, PH.D., is Senior Director, Xcenda, Palm Harbor, FL. CHRIS KOZMA, PHD., is. Independent Consultant and Adjunct Professor, University of South Carolina. DILESH DOSHI, PHARMD,, is Director, Regional Outcomes. Research; and MARCIA F. T. RUPNOW, PHD., is Director, Outcomes. Research (Neurology), Ortho-McNeil Janssen Scientific Affairs, LLC, Titusville, NJ.

Address correspondence to Dr. Bramley at Xcenda—West Region, 1528 Preston Street, Salt Lake City, UT 84108 (tbramley{at}xcenda.com).

Purpose. The frequency of potential drug–drug interactions (DDIs) between antiepileptic drugs (AEDs) and other (non-AED) medications in Medicaid patients taking newer AED monotherapy, older AED monotherapy, and combinations of AED treatment was studied. Methods. A retrospective, observational study was conducted using administrative claims obtained from South Carolina Medicaid. Patients were included in the analysis if they (1) had at least one prescription for an AED between January 1, 2004, and December 31, 2004, (2) were taking a specific AED for at least 60 days, (3) had at least one epilepsy diagnosis during the 6 months before or during the enrollment period, and (4) were enrolled in Medicaid for at least 11 of the 12 months of the follow-up period. Possible DDI exposure was defined as 10 days of overlap between an AED and a non-AED known to have the potential to cause a clinically relevant interaction. Results. A total of 4955 patients met the inclusion criteria. Approximately 45% of patients receiving monotherapy with an older AED had a potential DDI, compared with 3.9% receiving a newer AED. An average of 0.08 potential DDI per year of exposure occurred in the newer AED monotherapy cohort compared with 1.18 in the older AED monotherapy cohort. The most common potential interaction category was a decreased concentration of the non-AED. Conclusion. Older AEDs were associated with a greater likelihood of a potential DDI than were newer AEDs. Further research is needed to elucidate the relationship between the occurrence of potential DDIs and actual clinically relevant consequences. Index terms: Anticonvulsants; Blood levels; Drug interactions; Epilepsy; Health benefit programs