A 45-year-old woman with chronic back pain has been on morphine 30 mg twice daily for 8 months. She now reports constipation for the past 3 weeks (bowel movements only once every 4–5 days, hard stools, abdominal bloating) unresponsive to increased dietary fibre and water intake. She is unwilling to reduce opioid dose due to inadequate pain control. What is the most appropriate next step in management?