A 58-year-old man with metastatic pancreatic cancer presents to the palliative care clinic with severe, uncontrolled pain (8/10 on visual analogue scale) despite being on paracetamol 1 g QID and ibuprofen 400 mg TDS for the past 3 weeks. On examination, he appears cachectic but alert and oriented. His respiratory rate is 16/min, oxygen saturation 98% on room air, and blood pressure 128/82 mmHg. The palliative care physician decides to initiate opioid therapy. Which of the following opioids is the most appropriate first-line choice for this patient?