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.
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