Intravenous Methadone in the Management of Chronic Cancer Pain: Safe and Effective Starting Doses when Substituting Methadone for Fentanyl

Background: Patients often are rotated from other opioids to methadone when side effects occur before satisfactory analgesia is achieved. Various strategies have been proposed to estimate safe and effective starting doses of methadone when rotating from morphine and hydromorphone; however, there are no guidelines for estimating safe and effective starting doses of methadone when rotating from fentanyl.

Methods: The authors prospectively observed 18 consecutive patients experiencing chronic pain from cancer who underwent opioid rotation from intravenous patient-controlled analgesia (PCA) with fentanyl to intravenous PCA with methadone. Patients were switched from fentanyl to methadone because of uncontrolled pain associated with sedation or confusion. A conversion ratio of 25 microg/hour of fentanyl to 0.1 mg/hour of methadone was used to calculate the initial dose of methadone in all patients.

Results: Mean pain scores decreased from 8.1 to 4.8 on Day 1 after the switch and to 3.22 on Day 4 after the switch. Mean sedation scores were 1.5 before the switch and 0.44 and 0.16 on Days 1 and 4, respectively. Among the 6 patients who experienced confusion while on fentanyl before the switch, 5 improved within 2 days of the switch. None of the patients experienced toxicity from methadone.

Conclusions: On the basis of this preliminary study, the authors suggest that when switching from intravenous fentanyl to methadone a conversion ratio of 25 microg/hour of fentanyl to 0.1 mg/hour of methadone may be safe and effective.