Adverse drug reactions can be broadly defined as any undesirable response associated with therapeutic drug use. A simple and clinically useful classification is to divide adverse events into those that are dose-dependent and largely predictable from the known pharmacologic properties of the compound in question, and those that are dependent on characteristics unique to susceptible individuals, or idiosyncratic in nature.

The long term objective of this research is to characterize the mechanisms responsible for the pathogenesis of idiosyncratic hypersensitivity reactions in children, particularly those involving carbamazepine and other aromatic anticonvulsants.

The study is divided into two phases. Phase 1 of the study involves collecting urine from 50 patients taking CBZ therapeutically. Participants will be asked to provide a spot urine sample during routine health visits. The urine will be analyzed for the presence of CBZ and its metabolites. In Phase 2 of the study, urine will be collected from patients taking either CBZ or VPA therapeutically. If blood samples are drawn from these patients for medical purposes not related to this study the residual blood sample will be recovered before it is discarded for use in genotyping analysis. Participants will be asked to provide a urine sample covering one complete dosing interval of CBZ or VPA (preferably overnight). Patients will also be followed longitudinally, with urine collections at each clinic visit over at least a two year period.