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.