To the best of our knowledge, this is the first study to examine the pharmacogenetic impact of HTR2C –759C>T, ABCB1 1236C>T, ABCB1 2677G>T/A, and ABCB1 3435C>T polymorphisms on overweight/obesity in Thai children and adolescents with ASD treated with risperidone. The gene discussed is ABCB1; the disease is obesity disorder.