Body weight (BW) gain may be induced by perampanel (PER) administration, similar to the well-known adverse effects of valproic acid and gabapentin. Intellectual disability (ID) and serum PER concentration may be risk factors of BW gain. This study investigated how ID and serum PER concentration are associated with PER-induced BW gain. Subjects were 76 patients with epilepsy (41 men, aged 16 to 70 years). All patients were divided by intelligence quotient (IQ) into no ID (IQ 70, n = 24), mild to moderate ID (70 IQ 35, n = 31), and severe to profound ID (IQ