Wednesday, July 22, 2009

Dosing Ratios: Morphine-to-Methadone Rotation

Effective dosing ratios when switching opioid analgesic therapy from morphine to methadone in cancer patients (as well as for chronic pain) vary widely. Researchers in Spain assessed 54 patients with cancer undergoing opioid rotation to clarify factors that influenced the morphine-to-methadone equianalgesic dose ratio (MMEDR) at Day 10 after the switch. Reasons for switching opioids were uncontrolled pain (10 patients) or side effects (with or without pain, 44 patients).

Median prior morphine dose for the entire sample was 220 mg/day (range: 30–1000 mg/d). Initial MMEDR was 5:1 or 10:1 in 82% and 18% of patients, respectively. The stable median MMEDR at Day 10 was 5:1 (range: 2:1–15:1). Multiple linear regression analysis showed that the reason for switching (pain vs. side effects) and prior morphine dose (less than 300 mg/d vs. 300 or more mg/d) were significantly (p < .001) associated with the stable MMEDR. Therefore, these are predictive factors for more accurately selecting an appropriate conversion ratio (as shown in the Table below).

Caution: These ratios apply only to opioid-tolerant patients and are somewhat lower than those reported elsewhere in the literature. The same conversion factors should NOT be used in reverse, as when converting from methadone to morphine.

Reference: Benítez-Rosario MA, Salinas-Martín A, Aguirre-Jaime A, Pérez-Méndez L, Feria M. Morphine-methadone opioid rotation in cancer patients: analysis of dose ratio predicting factors. J Pain Symptom Manage. 2009(Jun);37(6):1061-1068.
Also see…
> Rotation From Other Opioids to Methadone Reviewed.
Pain-Topics Updates, 2008;16.
> Toombs JD. Oral Methadone Dosing for Chronic Pain: A Practitioner's Guide.
Pain Treatment Topics, 2008(Mar).