Absolute bioavailability of paracetamol after oral or rectal administration in healthy volunteers

The absolute bioavailability of paracetamol (Tachipirana®) administered in single doses orally and rectally was studied in nine healthy adult volunteers. The results of the rapid intravenous injection of 600 mg show that the kinetics of paracetamol can be represented by an open two-compartment model with introduction and elimination occurring in the central compartment. The oral administration shows an absolute bioavailability of 60-70% independent of the pharmaceutical form and gastric contents. The rectal administration shows a lower absolute bioavailability (of about 30-40%) substantially independent of dose in the range under consideration. The pharmaceutical form, food and the route of administration modify the plasma concentration-time curves of free paracetamol and of its main metabolites. A moderate inter-individual variation in the kinetics and the bioavailability was observed.