A 58-year-old man with a 3-year history of stable angina presents to the cardiology clinic. He reports chest discomfort on exertion that is relieved within 5 minutes of rest. His current medications include aspirin and atorvastatin. On examination, blood pressure is 145/92 mmHg and heart rate is 78/min. Resting ECG is normal. He is started on a long-acting nitrate (isosorbide mononitrate 60 mg once daily) and a beta-blocker (metoprolol 50 mg twice daily). After 2 weeks, he reports excellent symptom control. However, he now complains of a throbbing headache that occurs shortly after taking his morning dose of nitrate, though the headache resolves within 1–2 hours. What is the most appropriate management strategy?