The Therapeutic Equivalence of IM and PO Administration of Morphine – 1:3 or 1:6?

Abstract

When alternating between the intramuscular and oral routes of morphine adminstration, dosage adjust-ments must be made according to differences in rel-ative analgesic potency. While study has shown that six times as much morphine is required orally as intramuscularly to obtain the same total analgesic effect, anecdotal experience suggests that three times as much drug is required orally as is required parenterally. The following examines the arguments both for and against the two approaches to dose adjustment when alterations in route of administration are indicated. There is no compelling reason to change the use of the 1:3 ratio in cancer-related pain. The original experimental ratio of 1:6 is only an apparent inconsistency in that the conditions of dosing and other factors are most likely responsible for this apparent discrepancy. It will only be with a well-controlled study, however, conducted under appropriate conditions, that the 1:3 ratio can be confirmed.