- Subjects must be on a stable dose regimen of at least 1, but no more than 3 AEDs (concurrent stable Vagus Nerve Stimulation (VNS) is not counted as an AED). The VNS must have been in place for at least 6 months prior to study entry. The dosage of concomitant AED therapy and the settings of the VNS must be kept constant for a period of at least 4 weeks prior to entry into the Baseline Period