A 14-year-old boy is being treated for behavioral difficulties at home and at school and has been diagnosed with ADHD. He has been skipping class and often comes home smelling of cigarette smoke. His parents report he has developed sudden facial twitches and makes more frequent repetitive guttural sounds, especially since starting a new medication. His drug screen is negative for unprescribed substances of abuse. Which of the following is the most likely cause of his facial twitches and guttural sounds?