The initiation of methadone, a known effective analgesic for cancer pain, is complex. The existing protocols are often inadequately described; therefore, a classification of literature is needed. We reviewed and classified the recent literature on methadone initiation protocols in cancer patients experiencing severe pain. To provide a new classification of initiation protocols, based on a critical literature review. The MEDLINE database was searched for articles published until March 25, 2021, using the terms "cancer pain," "methadone," "methadone introduction," or "methadone initiation." The search was limited to human studies, randomized controlled trials (RCTs), other clinical trials, meta-analyses, and case reports. Selected articles were assessed for initiation details (rapid or progressive), administered dose (fixed rescue dose or ), and dose calculation (fixed or progressive ratios using morphine equivalent daily dose [MEDD] for daily or unitary dose). Twenty-four publications that met our inclusion criteria were analyzed. No large-scale prospective double-blind RCTs with robust design were identified. Most studies assessed relatively small numbers of patients. Eight initiation types were identified, of which three involved seven "high quality" studies: "rapid switch-fixed doses and rescue dose-progressive daily ratio," "progressive switch-fixed dose and rescue dose-progressive daily ratio," and "rapid switch--fixed ratio for unitary dose" protocols. This classification provides the latest information on methadone initiation protocols. The total daily dose of methadone varied largely across protocols. We recommend a maximal daily methadone dose of 100 mg (3 doses of 30 mg or 5 doses of 20 mg) for MEDD <500 mg, when the two "" protocols are used. Further clinical research on this topic is warranted.