A 38-year-old woman is 6 hours post-op following open abdominal hysterectomy. She is on PCA morphine with demand dose 1.5 mg, lockout interval 8 minutes, and 4-hour limit of 25 mg. She has received 18 mg in the past 4 hours and reports VAS 4/10 pain. Her respiratory rate is 14/min, SpO₂ 94% on room air, and she is drowsy but arousable. She complains of nausea and has vomited twice. What is the most appropriate management?