INTRODUCTION: As of 1st June 2005 it is possible to market generic epilepsy drugs with the active substance lamotrigine because the patent on the original product Lamictal had expired. We wanted to investigate if generic lamotrigine leads to larger variations in plasma concentrations of lamotrigine. MATERIALS AND METHODS: The citizens of the county of Vejle treated with lamotrigine were identified and classified in three groups according to the medication used after patent expiry: original brand only, generic brand only, or both generic and original brand. The defined daily dose (DDD) of lamotrigine was calculated for each patient and the data collected for each class. Thereafter a data search was carried out in the database from the department of Clinical Biochemistry in order to find corresponding patient levels of lamotrigine in plasma. RESULTS: The median plasma concentrations of lamotrigine before and after patent expiry for patients treated with the original brand only were 21.0 mmol/L before and 22.0 mmol/L after. For patients who changed their treatment from the original to the generic brand the concentrations were 15.5 mmol/L before and 20.5 mmol/L after. For patients who changed their treatment from the original to a mixture of original and generic brand the concentrations were 17.0 mmol/L before and 19.5 mmol/L after. The median of drug use of lamotrigine before/after patent expiry for patients mentioned in the same order was 167.5 DDD before and 141 DDD after (original after); 156 DDD before and 161 DDD after (generic only) and 147.5 DDD before and 160.5 DDD after (generic/original after). CONCLUSION: Generic lamotrigine does not lead to changes in plasma concentrations or larger variation in plasma concentrations. Consequently, there is no evidence to support that the effect is different when comparing the original brand and generic brands of lamotrigine. Further documentation can only be obtained by registering the numbers of seizures and side effects in a study comparing generic lamotrigine and the original brand.