The research hypothesis of this study is: In patients with moderate to severe cancer related pain, the addition of low dose methadone to an existing opioid significantly reduces pain severity compared to low doses morphine.

Inclusion Criteria:

• Patients diagnosed of cancer-related pain of more than 3 months duration
• Pain relief during the last week rated unsatisfactory by the patient
• Pain severity during the last week rated moderate (between 4 and 7/10)
• Analgesic therapy must have been stable for 7 days
• Able to understand English or French
• Willing and able to give written informed consent

Exclusion Criteria:

• Patients who are currently receiving or have received methadone as analgesic in the last 6 months
• Contraindication to receive methadone (allergy, QTc segment on the ECG>450msec, concurrent treatment with medication that could increase methadone's effects)
• Patients presenting with changes in their cancer status/treatment with potential effects on their pain severity, during the duration of the trial (eg: new metastasis, indication for radiotherapy)
• Patients whose life expectancy is shorter than 2 months
• Patients with cognitive impairment presenting with difficulties understanding the trial and completing the research questionnaires
• Pregnant or lactating women (women of childbearing potential must have negative pregnancy test)

1. Demographic, cancer status, pain medication, symptom severity and side effects profile will be collected for each participant throughout the study
2. Data will be collected by clinical interview with research nurse
3. Data will be available to researchers immediately