the invention relates to the use of cannabidiol , at a dose of greater than 300 mg / day , in combination with a standard anti - epileptic drug which acts via sodium or calcium channels , for use in the treatment of epilepsy . the saed is preferably one which • modifies low - threshold or transient neuronal calcium currents , or • reduces high - frequency neuronal firing and sodium - dependent action potentials and enhances gaba effects . preferred saeds are ethosuximide and valproate .