Dose ratios between high dose oral morphine or equivalents and oral methadone

Methadone is a commonly used opioid in hospice and palliative care for patients with refractory pain. Various methadone dose conversion methods utilize progressively higher morphine equivalent dose (MED) to methadone dose ratios to compensate for increased methadone potency with escalating opioid doses.

The purpose of this study was to determine the dose ratio between equianalgesic doses of high dose oral morphine (daily doses >1200 mg morphine or MED) and oral methadone.

This study was a retrospective chart review of 324 patients who received methadone at Strong Memorial Hospital or the associated outpatient clinic during a nine-month period in 2011. Ten patients met the study inclusion and exclusion criteria. A Wilcoxon signed-rank test was used to compare the pain scale scores. The Spearman correlation coefficient was used to assess level of correlation between morphine dose and methadone dose.

Patients rotated to methadone from high opioid doses had a two-point improvement in median pain scale scores after conversion (p=0.039). However, there was no correlation identified for patients taking daily doses >1200 mg oral morphine or MED prior to methadone rotation (p=0.19). There were no reported methadone adverse effects during the study.

No correlation was identified between high MED doses and methadone at dose stabilization after opioid rotation. A fixed maximum methadone dose of 30 mg/day produced clinically meaningful improvements in pain scores without adverse drug effects. Caution should be exercised before considering rotation to methadone doses higher than 30 mg/day in a patient receiving >1200 mg oral MED/day.