This is an open label study to evaluate the safety of ZX008 (fenfluramine) in patients with Dravet syndrome (DS) or Lennox Gastaut syndrome (LGS) who are being administered cannabidiol (CBD).

Key Inclusion Criteria:

• Subject has a clinical diagnosis of DS or LGS, where convulsive seizures (generalized tonic clonic seizures, tonic seizures, atonic seizures, tonic/atonic seizures, focal seizures with clear observable motor signs) are not completely controlled by current AEDs.
• All medications or interventions for epilepsy (including ketogenic diet [KD] and vagal nerve stimulation [VNS]) must be stable for at least 4 weeks prior to first ZX008 dose and are expected to remain stable throughout the study.
• Subjects must be taking a stable dose of CBD for at least 4 weeks prior to first ZX008 dose.
• Subject's source of CBD is expected to be consistent for at least 3 months during study participation.

Key Exclusion Criteria:

• Subject has current or past history of cardiovascular or cerebrovascular disease, myocardial infarction or stroke.
• Subject with current cardiac valvulopathy or pulmonary hypertension that the Investigator, parent/caregiver, Independent Cardiac Advisory Board, Independent Data and Safety Monitoring Committee, or sponsor deems clinically significant.
• Subject has a current or past history of glaucoma.