A 38-year-old male with obsessive-compulsive disorder is started on fluvoxamine 100 mg daily. He drinks 4-5 cups of coffee daily and also takes theophylline for asthma. After 2 weeks on fluvoxamine, he reports jitteriness, insomnia, heart palpitations, and nausea. His theophylline level returns significantly elevated above the therapeutic range. Which of the following best explains this clinical picture?