Authors' Response

Abstract

To the Editor: The authors wholeheartedly concur that it is wrong to recommend a change in established conversion ratios based on a single case study. This case report highlights the controversy surrounding the relative oral milligram potency ratio of morphine to oxycodone, but does not resolve the issue. A relative oral milligram potency ratio of 2:1 has been cited by the pharmaceutical industry (Purdue Pharma, L.P., Norwalk, CT) and others.1Houde R. The use and misuse of narcotics in the treatment of chronic pain. In: Bonica JJ, ed. Advances in Neurology. New York: Raven Press, 1974;4:527–538.Google Scholar, 2Kaiko R. Lacouture P. Hopf K. et al.Analgesic onset and potency of oral controlled-release (CR) oxycodone and CR morphine.Clin Pharmacol Ther. 1996; 59 ([abstract]): 130Google Scholar, 3Kaiko R.F. The use of controlled release opioids.in: Parris W.C.V. Cancer pain management principles and practice. Butterworth–Heinemann, Boston1997: 69-90Google Scholar In clinical practice, a relative oral milligram potency ratio of 1:1 is commonly used.4Glare P.A. Walsh D. Oxycodone a substitute for morphine in cancer pain management.Palliat Med. 1992; 6 ([letter]): 79-80Crossref Scopus (7) Google Scholar Dr. Miller critiques our support for a 1:1 relative oral milligram potency ratio based on the patient's conversion from oral morphine to oral oxycodone. Our support for a 1:1 relative oral milligram potency ratio is not derived from the patient's conversion from oral morphine to oral oxycodone at a time when she had poor pain control. Rather, this ratio is favored based on the patient's conversion from oral oxycodone to parenteral morphine using a 3:1 relative oral:parenteral milligram potency ratio and back again to oral oxycodone at the same dose. In this setting, the patient maintained stable (and excellent) analgesia. As previously hypothesized, potential reasons for disparities in the relative milligram potency ratio of these two drugs may relate to the use of a single dose block crossover design in earlier studies and/or to change in oral to parenteral morphine equivalencies with the development of opioid tolerance.5Zhukovsky D.S. Walsh D. Doona M. The relative potency between high dose oral oxycodone and intravenous morphine a case illustration.J Pain Symptom Manage. 1999; 18: 53-55Abstract Full Text Full Text PDF PubMed Scopus (20) Google Scholar We reiterate that apparent differences in the relative milligram potency ratios as determined from clinical practice,5Zhukovsky D.S. Walsh D. Doona M. The relative potency between high dose oral oxycodone and intravenous morphine a case illustration.J Pain Symptom Manage. 1999; 18: 53-55Abstract Full Text Full Text PDF PubMed Scopus (20) Google Scholar pharmaceutical company (Purdue Pharma, L.P., Norwalk, CT) and other5Zhukovsky D.S. Walsh D. Doona M. The relative potency between high dose oral oxycodone and intravenous morphine a case illustration.J Pain Symptom Manage. 1999; 18: 53-55Abstract Full Text Full Text PDF PubMed Scopus (20) Google Scholar, 6Glare P.A. Walsh T.D. Dose-ranging study of oxycodone for chronic pain in advanced cancer.J Clin Oncol. 1993; 11: 973-978PubMed Google Scholar, 7Kalso E. Vainio A. Morphine and oxycodone hydrochloride in the management of cancer pain.Clin Pharmacol Ther. 1990; 47: 639-646Crossref PubMed Scopus (247) Google Scholar, 8Houde R.W. Systemic analgesics and related drugs narcotic analgesics.in: Bonica J.J. Ventafridda V. Advances in pain research and therapy, Vol 2. Raven Press, New York1979: 263-273Google Scholar recommendations be resolved by repeated dose controlled studies of oral oxycodone to oral morphine in cancer pain populations to clarify the clinically important issues of relative milligram potency ratios, therapeutic efficacy, and side effects. This would allow prescribers to make rational choices between the two drugs, and to assess the pharmacoeconomic impact of that decision. The latter is significantly affected by the conversion ratio employed. This is a critical issue in hospice practice, where large volumes of opioids are prescribed under severely restricted reimbursement.