326 Letters Vol. 19 No. 5 May 2000

Re: The Relative Potency Between High-Dose Oral Oxycodone and Intravenous Morphine

To the Editor:The case study by Zhukovsky et al.

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describesa patient who did not gain adequate pain con-trol despite treatment with 1200 mg of oralmorphine a day. The authors converted the pa-tient to a daily dose of 1200 mg of oral oxyc-odone, which provided adequate analgesia. Onthis basis they conclude that the conversion ra-tio is 1:1.

An opioid conversion ratio should provide aguide for the clinician wishing to convert oneopioid to an equianalgesic dose of another.Since the doses of opioids described in theabove case study were not equianalgesic, noconclusion may be made in regard to the con-version ratio.

Oxycodone and morphine have been avail-able in oral formulation for many years in theUSA and much clinical experience of convert-ing patients from one to the other exists. A po-tency ratio of 1:2 (morphine:oxycodone) hasbeen used for many years,

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and this ratio hasbeen supported with data from a clinical trial(

n

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154) specifically designed to study this is-sue.

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It would be wrong to recommend achange in the 1:2 conversion ratio based on asingle case study.

Allan Miller, MDNapp Pharmaceuticals, Ltd.Cambridge, United Kingdom

PII S0885-3924(00)00134-2

References

1. Zhukovsky DS, Walsh D, Doona M. The relativepotency between high dose oral oxycodone and in-travenous morphine: a case illustration. J PainSymptom Manage 1999;18:53.

2. Houde R. The use and misuse of narcotics in thetreatment of chronic pain. In: Advances in Neurol-ogy, Vol 4. New York: Raven Press, 1974:527–536.

3. Kaiko R, Lacouture P, Hopf K, et al. Analgesic on-set and potency of controlled release oxycodoneand CR morphine. Clin Pharmacol Ther 1996;59:130 (abstract P1-4).

Authors’ Response

To the Editor:The authors wholeheartedly concur that it is

wrong to recommend a change in establishedconversion ratios based on a single case study.This case report highlights the controversy sur-rounding the relative oral milligram potencyratio of morphine to oxycodone, but does notresolve the issue. A relative oral milligram po-tency ratio of 2:1 has been cited by the pharma-ceutical industry (Purdue Pharma, L.P., Nor-walk, CT) and others.

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In clinical practice,a relative oral milligram potency ratio of 1:1is commonly used.

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Dr. Miller critiques oursupport for a 1:1 relative oral milligram po-tency ratio based on the patient’s conversionfrom oral morphine to oral oxycodone. Oursupport for a 1:1 relative oral milligram po-tency ratio is not derived from the patient’sconversion from oral morphine to oral oxyc-odone at a time when she had poor pain con-trol. Rather, this ratio is favored based on thepatient’s conversion from oral oxycodone toparenteral morphine using a 3:1 relative oral:parenteral milligram potency ratio and backagain to oral oxycodone at the same dose. Inthis setting, the patient maintained stable (andexcellent) analgesia. As previously hypothe-sized, potential reasons for disparities in therelative milligram potency ratio of these twodrugs may relate to the use of a single doseblock crossover design in earlier studies and/or to change in oral to parenteral morphineequivalencies with the development of opioidtolerance.

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We reiterate that apparent differ-ences in the relative milligram potency ratiosas determined from clinical practice,

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phar-maceutical company (Purdue Pharma, L.P.,Norwalk, CT) and other

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recommendationsbe resolved by repeated dose controlled stud-ies of oral oxycodone to oral morphine in can-cer pain populations to clarify the clinically im-portant issues of relative milligram potencyratios, therapeutic efficacy, and side effects.This would allow prescribers to make rationalchoices between the two drugs, and to assessthe pharmacoeconomic impact of that deci-sion. The latter is significantly affected by theconversion ratio employed. This is a critical is-sue in hospice practice, where large volumes of