gms | German Medical Science

Introduction: Generic substitution of antiepileptic drugs is controversially discussed, and rebate contracts between pharmaceutical manufacturers and statutory health insurance funds (SHI) may have increased the problem. The aim of this study was to quantify generic switching in patients treated with valproic acid (VPA), and to compare switch rates from 2006 to 2009, i.e. before and after rebate contracts were implemented.

Methods: Data were extracted from the DAPI database, containing drug dispensing data at the expense of the SHI from more than 80% of German community pharmacies. For each year, patients with an index prescription (last prescription) of VPA in the first quarter of each year were followed until the first generic switch or censoring (due to interruption or discontinuation of VPA treatment or Dec 31 of the year). Incidence rates were calculated for different VPA formulations (immediate release IR, extended release ER or Orfiril long^® ORL), and analysis was stratified by VPA dosage strength, VPA pretreatment duration and comedication.

Results: Incidence rates for generic switching of VPA in 2006 were 21.8, 29.2 and 8.2 per 100 person-years (PY) for patients treated with IR, ER and ORL, respectively. In 2008, after rebate contracts came into effect, incidence rates peaked at 45.4 and 51.6 for IR and ER formulations, but not for ORL products (5.7 per 100 PY). Incidence rates were slightly increased in the stratum of higher VPA dosage strength, in patients with VPA pretreatment duration of 1–2 years versus 0–1 and >2 years as well as in patients pretreated with further antiepileptic drugs.

Discussion and conclusion: These results show that generic substitution of VPA was frequent already in 2006 and has further increased after rebate contracts came into effect. However, this effect is not uniform across different VPA formulations, since generic switches for patients treated with ORL remain constantly low. Incidence rates from stratified analyses suggest that generic switching is not considerably avoided in patients receiving higher VPA dosage strength or antiepileptic drug combinations as well as in patients with long VPA treatment duration. In subsequent analyses, the effect of generic switching on the necessity to prescribe further antiepileptic drugs (as a proxy for treatment failure) will be studied.