Oral opioids in the treatment of cancer pain.

Persistent severe cancer pain should be treated with opioid drugs, principally morphine. It can be administered orally, rectally and parenterally. Morphine is metabolised in the liver mainly to glucuronides, of which morphine-6-glucuronide is a powerful analgesic. Oral morphine should be administered regularly and in individualized doses. The use of morphine is frequently accompanied by adverse effects such as constipation, nausea, vomiting and sedation. Management of these is critical for successful pain treatment. Although alternatives are available none has any clear advantage over morphine in cancer pain, and should be reserved for special situations. Oral morphine is successful in more than 90% of cancer pain patients. Slow release morphine sulphate tablets (MS Contin) are often the best choice. For the few patients who need parenteral medication, continuous subcutaneous morphine sulphate infusion is generally the most suitable. Some pains are morphine resistant, especially those due to nerve injury. In these cases pain is best treated with tricyclic antidepressants and/or anticonvulsants.