Provided herein is a method of treating a selected patient population, wherein the patient population is selected based on a determination that the patients have previously been non-responsive when treated with cannabidiol. In some embodiments, the method comprises selecting the patient based on a previously failed treatment with cannabidiol, based on lack of efficacy or tolerability. Pharmaceutical compositions and formulations for use in practicing the subject methods are also provided. The method comprises identifying a population of patients diagnosed with Dravet syndrome who were found previously to have been non-responsive when treated with cannabidiol. The selected population of patients is then treated by administering, to each identified patient, a therapeutically effective dose of fenfluramine or a pharmaceutically acceptable salt, base, acid or amine thereof, and repeating the administering over a period of a day or days, or over a period of weeks, months or years, until the patient exhibits a reduction from baseline in convulsive seizure frequency.
