Equianalgesic dose ratios for opioids: A critical review and proposals forlong-term dosing

Clinicians involved in the opioid pharmacotherapy of cancer-related pain sh ould be acquainted with a variety of opioids and be skilled in the selectio n of doses when the type, (of opioid or route of administration needs chang ing. The optimal dose should avoid underdosing or overdosing, both associat ed with negative outcomes for the patient. Although equianalgesic dose tabl es are generally used to determine the new doses in these circumstances, th e evidence to support the ratios indicated in these tables largely refers t o the context of single dose administration. The applicability of these rat ios to the setting of chronic opioid administration has been questioned. A systematic search of published literature from 1966 to September 1999 was c onducted to critically appraise the emerging evidence on equianalgesic dose ratios derived from studies of chronic opioid administration. There were s ix major findings: 1) there exists a general paucity of data related to lon g-term dosing and studies are heterogeneous in nature; 2) the ratios exhibi t extremely wide ranges; 3) methadone is more potent than previously apprec iated; 4) the ratios related to methadone are highly correlated with the do se of the previous opioid; 5) the ratio may change according to the directi on the opioid switch; and 6) discrepancies exist with respect to both oxyco done and fentanyl. Overall, these findings have important clinical implicat ions for clinicians and warrant consideration in the potential revision of current tables. The complexity of the clinical context in which many switch es occur must be recognized and also appreciated in the design of future st udies. (C) U.S. Cancer Pain Relief Committee, 2001.