A 38-year-old female patient is 4 hours post-op following laparoscopic cholecystectomy. She is on intravenous PCA with fentanyl (10 mcg bolus, 6-minute lockout, 4-hour limit 100 mcg). She complains of nausea and has vomited once. She has pressed the PCA button only 3 times in the last hour. Her vital signs are: BP 118/76, HR 82, RR 14, SpO₂ 97% on room air. She is drowsy but arousable. What is the most appropriate next step in management?