The economic consequences of generic substitution for antiepileptic drugs in a public payer setting: the case of lamotrigine.

Generic substitution of antiepileptic drugs (AEDs) may increase pharmacy utilization, thus counterbalancing per-pill savings. The purpose of our study was to analyze the economic impact of government-mandated switching from branded to generic lamotrigine. Patients in a Canadian public pharmacy claims database using branded lamotrigine (Lamictal GlaxoSmithKline, UK) in 2002 converted to generic lamotrigine in 2003 and were observed from July 2002 to March 2006. Patients used branded lamotrigine for >or=90 days pre-generic entry and had >or=1 claim for generic lamotrigine post-generic entry. For the generic period, observed per-patient monthly drug costs were calculated as the sum of costs for lamotrigine, other AEDs, and non-AEDs. Expected per-patient drug costs were estimated assuming lamotrigine dose and other prescription drug utilization in the generic period were identical to those observed during the brand period. Differences between observed and expected costs were compared. Among 1,142 branded lamotrigine users, overall average monthly drug costs per person were expected to decrease by $30.55 due to lower pill costs. Instead, they fell by $11.98 from the brand to the generic periods (p < 0.001). Because of dosage changes, lamotrigine costs decreased by $29.92 instead of the anticipated $33.87 (p < 0.001). Increased pharmacy utilization caused other AED costs to rise by $6.29 versus the expected $0.36 (p < 0.001), while non-AED drug cost increased by $11.64 rather than by $2.95 (p < 0.001). We concluded that conversion to generic lamotrigine resulted in lower than expected cost savings. Further research is necessary to determine whether this is due to reduced effectiveness and/or tolerability. Payers may weigh smaller-than-expected cost reductions against a possible decrease in effectiveness to assess the relevance of mandatory generic switching of lamotrigine.