A 42-year-old woman undergoes elective total abdominal hysterectomy under general anesthesia. Postoperatively, she is prescribed intravenous patient-controlled analgesia (PCA) with morphine (1 mg bolus, 10-minute lockout interval, 4-hour limit of 30 mg). On postoperative day 1, she complains of inadequate pain relief despite pressing the PCA button frequently. Her vital signs are stable (BP 128/82, HR 88, RR 16, SpO₂ 98% on room air), and she is alert and oriented. The nursing staff notes she has been pressing the button approximately every 2 minutes. What is the most appropriate next step in management?