Psychiatryniche
A 46-year-old man with schizophrenia has taken clozapine 400 mg daily for four years without a dose change. Over the past ten days he has become drowsy in the afternoons, soaks his pillow with saliva overnight and has twice stumbled getting out of a chair; his wife saw two brief jerks of both arms yesterday. Three weeks ago he joined a workplace program, started varenicline and has not smoked since, and he has cut down to one coffee a day. He is afebrile, his chest is clear, and last week's neutrophil count was normal. Which one of the following best explains his new symptoms?