Equianalgesic dose/ratio between methadone and other opioid agonists in cancer pain: comparison of two clinical experiences.

BACKGROUND: Oral methadone is considered to be a valid opioid analgesic alternative to morphine and hydromorphone in treating cancer pain. However, the use of methadone could be complicated by the limited knowledge of the equianalgesic dose/ratio with the other analgesic opioids when switching in tolerant patients. PATIENTS AND METHODS: In two Palliative Care Units, data collected regarding 88 advanced cancer patients with pain switched from different opioids to oral methadone were reviewed and compared with the aim of determining the equianalgesic dose ratio in relation to the dose of opioid previously administered.
RESULTS: The results of this retrospective study suggest that: (1) methadone is much more potent than previously described in literature, (2) the dose ratio between hydromorphone and methadone is higher than as suggested by equianalgesic tables, and (3) the ratio correlates with total opioid dose administered before switching.
CONCLUSIONS: The fact that methadone ratio is different according to the opioid dose used previously should be taken into careful consideration by the clinician in order to avoid severe toxicity or death during switchover. Prospective studies should be carried out in order to better define our findings.