 The primary objective is to provide high-quality evidence on potential risks associated with substitution of the currently taken AED product carbamazepine,valproic acid, topiramate, oxcarbazepine, levetiracetam or lamotrigine with an equivalent product, using as endpoint changes in serum drug levels at steady-state after substitution compared with baseline. Secondary objectives will be the assessment of inter-subject variability in serum drug concentration on an unchanged treatment schedule, and evaluation of potential short-term changes in seizure control and adverse events rate. 