 - Currently receiving monotherapy treatment with an antiepileptic drug AED at a stable dose for at least 28 days prior to the screening visit Visit 1. If the subject is taking a barbiturate e.g., phenobarbital, the dose must be stable for 3 months prior to the Screening Visit. Note: Subjects who have received previous adjunctive treatment but are currently taking one AED are eligible for enrolment. 