A 34-year-old male with treatment-resistant schizophrenia has been stable on clozapine 400 mg daily with a therapeutic serum level of 380 ng/mL. He develops comorbid OCD, and his outpatient psychiatrist starts fluvoxamine 50 mg daily. Three weeks later, he presents with excessive sedation, sialorrhea, tachycardia, and confusion. A repeat clozapine level returns at 910 ng/mL. Which of the following best explains this interaction?