At the blood-brain barrier, overexpression of the drug efflux transporter ABCC2 (also known as MRP2) has been proposed as a mechanism for impaired carbamazepine (CBZ) treatment response in epilepsy. However, investigation of the impact of ABCC2 polymorphisms on CBZ treatment efficacy has produced conflicting and inconclusive results. A series of in vitro cell efflux and plasma membrane vesicle uptake assays were undertaken to investigate whether CBZ was an ABCC2 substrate. In addition, the effect of three common ABCC2 polymorphisms, -24C>T, c.1249G>A and c.3972C>T, on the efficacy of CBZ in epilepsy (assessed using the clinical end points time to first seizure and time to 12-month remission from the SANAD (Standard and New Antiepileptic Drugs) trial) was determined. CBZ was found not to be a substrate for human ABCC2 in vitro. Clinically, no significant association was observed for the ABCC2 genetic variants and CBZ treatment outcomes. This comprehensive analysis does not support a role for ABCC2 in CBZ treatment efficacy.