Journal of Palliative Care

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.

Clinical use suggests that the oral-to-subcutaneous / intramuscular potency ratio for morphine sulphate when given regularly to cancer patients is of the order of 1:2 or 1:3. Published data indicate that the mean bioavailability after oral administration is 38%, ranging from 15% to 64%. Recent pharmacokinetic studies have highlighted the important contribution made by an active metabolite, morphine-6-glucuronide, to the total effect of morphine. This is several times more potent than morphine and, on chronic administration, is present in higher concentrations than morphine itself. The need for caution when prescribing morphine to patients with impaired renal function is stressed.

The following list arranged alphabetically provides names and addresses of those persons contributing to this issue of the Journal of Palliative Care. Note that affiliations and current mailing addresses may differ.