Abstract Objectives To assess the efficacy and safety profile of add-on cannabidiol (CBD) in patients with Lennox–Gastaut syndrome (LGS) and Dravet syndrome (DS) on clobazam and in the overall population of four randomized, controlled phase 3 trials. Methods Patients received plant-derived, highly purified CBD medicine (Epidiolex® in the USA; Epidyolex® in Europe; 100 mg/ml oral solution) at a dose of 10 or 20 mg/kg/day, or placebo for 14 weeks. A subgroup analysis of patients on clobazam and meta-analysis by syndrome were conducted. The primary endpoint was percentage reduction in primary seizure type during the treatment period. Results 396 patients with LGS (49% on clobazam)...