The bioavailability of rectally administered morphine.

Plasma concentrations of morphine were followed for 24 hours in eight patients after intravenous and rectal administration of 10 mg morphine chloride. The plasma levels of morphine were determined by a sensitive and specific radioimmunoassay based upon an extraction procedure which separates morphine from its major polar metabolites. The bioavailability of morphine after rectal administration was found to be 53.3 +/- 17.8% (mean +/- S.D.). Peak concentrations of 16.3 +/- 8.7 ng ml-1 were reached after 59 +/- 16 min. The study indicates that first pass elimination of morphine may be partially avoided by rectal administration.