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Introduction: The Japanese packaging instructions for methadone prohibit dose escalation within 7 days of administration initiation as this may result in overdose and subsequent adverse events. However, for terminal cancer patients, evaluation of the effects of methadone may be desirable within 7 days because they have limited prognoses. We aimed to determine the possibility of estimating the adequateness of methadone earlier than the 7th day by investigating the onset timing of analgesic effects and adverse events of methadone in Japanese terminal cancer patients., Methods: Japanese terminal cancer patients who started taking methadone in Ashiya Municipal Hospital were enrolled from January 1, 2013 to February 28, 2019. Verbal rating scale (VRS) scores on pain and adverse events before and after methadone administration (on days 3, 5, and 7) were retrospectively investigated from medical records., Results: We enrolled 25 patients, of which 20 (80.0%) received methadone until day 7. The VRS score (mean ± standard deviation) on pain was significantly reduced to 0.90 ± 0.55 on day 3, compared with 1.65 ± 0.67 before the administration of methadone (p < 0.05). The mean VRS scores did not differ significantly on days 3, 5, and 7. Additionally, of the 23 patients who received methadone until day 3, 20 (87.0%) showed an analgesic effect on day 3 and 17 (85.0%) received methadone without experiencing serious adverse events until day 7., Conclusions: The adequateness of methadone in Japanese terminal cancer patients could be determined before day 7, considering the high analgesia incidence and few adverse events 3 days after the methadone administration under careful observation by a physician experienced in methadone administration. However, as this is a preliminary study, the relationship between pharmacokinetic parameters and analgesic effects was not evaluated. Further studies involving pharmacokinetics and multicenter prospective studies are required to support these findings., Competing Interests: None, (Copyright © Japan Medical Association.)