Patients with cancer, and increasingly chronic non-cancer pain frequently require strong opioids for pain relief. Morphine is the first-line strong opioid of choice for these patients. While most achieve adequate analgesia with morphine, a significant minority either suffer intolerable side-effects, inadequate pain relief, or both. For these patients switching to an alternative opioid is becoming established clinical practice. However, the evidence for the effectiveness of opioid switching does not appear to be established.