Methadone is considered to be a highly effective drug for treating cancer pain. However, it has been identified that the complex pharmacokinetics may cause serious side effects such as respiratory depression and QT prolongation associated with overdose. in this study, pharmacists intervened in cases of methadone introduction for intractable cancer pain from the stage of studying the switching method; it was safely conducted from introduction in hospital to continuation in outpatient department. in addition to confirming respiratory status and drowsiness as side effect monitoring, ECG testing by a pharmacist using a simple electrocardiograph was performed;transition to the outpatient department was conducted safely. Pharmacists who understand the pharmaceutical characteristics, mechanism of action, and pharmacokinetics of drugs can contribute to drug therapy and improve patients' quality of life by intervening from a pharmaceutical perspective.