Drastic improvement in the rectal absorption profile of morphine in man

Rectal absorption of morphine HCl from aqueous vehicles at different pHs in man has been compared with an orally administered solution. Plasma concentrations of morphine were measured by electrochemical HPLC analysis after a single dose of 10 mg morphine HCl, in a cross-over study in 7 volunteers. Rectal absorption of morphine was dependent on pH, which could be explained as being due to pH partitioning. The absorption rate and bioavailability could be greatly improved, as compared to orally administered morphine, by adjusting the pH. It was concluded that a rectal solution adjusted to pH 7 to 8 provided an entirely adequate dosage form.