Efficacy and Safety of Ultra-low Dose Methadone as Adjuvant Analgesic in Cancer Patients With Pain

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)