Rectal versus oral absorption of codeine phosphate in man

Rectal absorption of codeine phosphate from various dosage forms was studied in man. The rectal dosage forms included aqueous solutions and fatty suppositories. A comparison was made with an orally administered solution. The plasma concentrations of codeine were measured by means of HPLC analysis after a single dose of 60 mg codeine phosphate in a cross-over study in 7 volunteers. Compared with oral dosing rectal absorption from an aqueous solution or a fatty suppository produced an almost identical plasma concentration profile with similar interindividual variations. Comparing the absorption rate characteristics it appeared that rectal absorption from an alkaline solution containing codeine phosphate proceeded significantly (P less than 0.05) more rapid than after oral dosing. No essential difference in bioavailability was observed between the various rectal and oral dosage forms.