Switching from Methadone to a Different Opioid: What Is the Equianalgesic Dose Ratio?

Introduction: Methadone (ME) is a highly effective opioid agonist used for difficult pain syndromes. However, in the management of cancer pain with strong opioids, rotation to a different opioid (opioid rotation) may be required because of side effects or poor pain control. Rotation from methadone to another opioid has received limited study and therefore may be difficult because of the absence of a uniformly accepted dose conversion ratio. Methods: Retrospectively reviewed consecutive medical records of patients undergoing an opioid rotation from methadone to an alternative opioid were evaluated. For inclusion, patients were required to have received methadone for at least 3 days and have reached stable dose of the alternative opioid(s) during the 7 days following. Stable dose was defined as a 30% or less change in opioid dose from one day to the next. Results: Records of 39 patients met inclusion criteria. Excluded from analysis were 5 patients who were restarted on methadone within 7 days, 2 with irregular opioid use resulting in negligible regular opioid doses post-switch, and 3 due to concerns about reliability of multiple routes used for fentanyl. Data from 29 patients, 10 female, mean age 48 +/- 14.4 years, were evaluable. The mean dose ratio for oral methadone to oral morphine equivalent daily dose (MEDD) was 1:4.7 (95% confidence interval [CI], 3.0-6.5; n = 16), and for intravenous (IV) methadone to MEDD was 1:13.5 (95% CI, 6.6-20.5; n = 13), p = 0.06. Methadone dose was significantly correlated to stable MEDD after switching opioids for both methadone IV and oral (Spearman = 0.86, p = 0.0001 and Spearman = 0.72, p = 0.0024), respectively. Mean day of achieving stable dose was day 2.5 +/- 0.2 for IV methadone and day 2.6 +/- 0.3 for oral methadone. Conclusion: These dose ratios are new findings that may assist in switching patients more safely to alternative opioids when side effects or pain problems occur when patients are receiving methadone. An important difference in analgesic potency appears to exist between IV and oral ME. Future research with prospective studies is required.