Evaluation of the Efficacy and Safety of Two Methadone Titration Methods for the Treatment of Cancer-related Pain With Inadequate Pain Relief or Intolerable Side Effects When Treated With Level 3 Opioids.

Inclusion Criteria: * Adult patient of at least 18 years of age * Patient suffering from cancer disease, undergoing chemotherapy treatment or not, hospitalised or requiring hospitalisation * Patient presenting nociceptive or mixed pains inadequately relieved with level 3 opioids (morphine, oxycodone, transdermal fentanyl, hydromorphone) with a numeric pain scale score ≥5 evidence-based or presenting intolerable side effects with level 3 opioids * Patient undergoing level 3 opioid treatment (morphine sulphate or morphine chlorhydrate or fentanyl or oxycodone or hydromorphone) * Patient presenting a good understanding of the study objectives and able to give his/her written consent * Patient able to communicate with the investigator or his representative * Patient available during the whole course of the study and agreeing the study requirements * Patient with Social Insurance * Patient having received patient's information form, orally informed and having signed the consent form Exclusion Criteria: * Patient in terminal cancer disease (life expectancy less than 2 months) according to investigator's judgement * Patient treated with a medication that may result in an interaction with methadone, such as: antiarrythmic treatments (Ia or III), erythromycin, spiramycin, intravenous vincamin * Patient receiving opioid treatment for any other reason than pain * Patient unable to swallow the study treatment * Patient presenting contra-indication to the use of methadone * Patient having a decompensated respiratory failure or a severe hepatic disease * Patient having a known hypersensitivity to methadone * Patient presenting QT interval prolongation on ECG results * Patient receiving a concomitant treatment with a morphine-type agonist-antagonist medication (pentazocine, buprenorphine, nalbuphine) or with sultopride, or with morphinic antagonist (naltrexone) * Patient treated by analgesic radiotherapy within 15 days before inclusion * Patient participating or having participated in another clinical trial with a new therapy within one month before inclusion * Patient with a history of substance abuse * For woman with childbearing potential: pregnancy or breastfeeding. * Forfeiture of freedom or under guardianship * Past history of suicidal attempts * Patient likely not to respect the study conditions and/or study discontinuation criteria according to investigator's judgement * Patient presenting any other medical condition or illness or clinically significant abnormal findings on the physical examination at screening that, in the opinion of the Investigator, make the patient unsuitable for the study or put the patient at additional risk.