Systemic opioid therapy for chronic cancer pain - Practical guidelines for converting drugs and routes of administration

A switch from one drug or route of administration to another is common practice during long term opioid therapy in patients with chronic cancer pain. A change in drug, which is known as 'opioid rotation' or 'sequential opioid trials,' has become an accepted strategy to address an initially poor response to a specific opioid. The approach derives from the observation that there is large individual variation in the pattern of adverse effects produced by different opioids, and that the balance between analgesia and adverse effects may be improved following a switch to an alternative drug. A change of route is usually driven by practical considerations, typically the loss of the oral route. Regardless of the indication, a well tolerated and effective conversion from one opioid or route of administration to another requires an understanding of administration guidelines based on relative potency. The equi-analgesic dose table provided in this review codifies the results of previous studies that have evaluated the relative potencies across drugs or routes. To use the information contained in this table, the clinician must understand its derivation, limitations and practical application. The equi-analgesic dose ratios largely represent average data from controlled single-dose studies in selected populations. These values are tentative in the setting of long term administration of the drugs to medically ill patients. The ratios also fail to account for the possibility of incomplete cross-tolerance, which would render the new drug more potent than anticipated, and the impact on administration of either unrelieved pain or a predisposition to opioid toxicity. To accommodate these uncertainties, guidelines for the use of the equi-analgesic dose table include a standard reduction in the calculated equi-analgesic dose, which is then further adjusted based on the clinical characteristics of the patient. The new dose is then the starting point for the process of dose titration. These guidelines must be further adapted for two special cases: a switch to methadone (which has been observed to yield a potency much greater than expected) and conversions that involve the transdermal fentanyl system.