Objective: Many previous studies of drug repurposing have relied on literature review followed by evaluation of a limited number of candidate compounds. Here, we demonstrate the feasibility of a more comprehensive approach using high-throughput screening to identify inhibitors of a gain-of-function mutation in the SCN8A gene associated with severe pediatric epilepsy. Methods: We developed cellular models expressing wild-type or an R1872Q mutation in the Nav1.6 sodium channel encoded by SCN8A. Voltage clamp experiments in HEK-293 cells expressing the SCN8A R1872Q mutation demonstrated a leftward shift in sodium channel activation as well as delayed inactivation; both changes are consistent with a gain-of-function mutation. We next developed a fluorescence-based, sodium flux assay and used it to assess an extensive library of approved drugs, including a panel of antiepileptic drugs, for inhibitory activity in the mutated cell line. Lead candidates were evaluated in follow-on studies to generate concentration-response curves for inhibiting sodium influx. Select compounds of clinical interest were evaluated by electrophysiology to further characterize drug effects on wild-type and mutant sodium channel functions. Results: The screen identified 90 drugs that significantly inhibited sodium influx in the R1872Q cell line. Four drugs of potential clinical interest—amitriptyline, carvedilol, nilvadipine, and carbamazepine—were further investigated and demonstrated concentration-dependent inhibition of sodium channel currents. Significance: A comprehensive drug repurposing screen identified potential new candidates for the treatment of epilepsy caused by the R1872Q mutation in the SCN8A gene.