The effect of activated charcoal on the bioavailability of piroxicam in man. The effect of single and multiple oral doses of activated charcoal (a.c.) on the plasma concentrations of piroxicam was investigated in a cross-over study in 6 healthy volunteers after oral and rectal doses of 20 mg piroxicam. 50 g a.c. swallowed 5 min after the oral administration of one capsule of piroxicam almost completely prevented the absorption of the drug. 70 g a.c. per day were given in multiple doses over the interval of 10-58 h after the oral and 2-58 h after the rectal administration of piroxicam. This treatment reduced the mean bioavailability of piroxicam by 41.7% (oral) or 48.8% (rectal), relative to the control. The apparent total clearance increased significantly (p less than 0.05) to 163.6% (oral) or 187.9% (rectal) of the control. Half-lives of elimination were reduced on the average from 40.2 h to 19.6 h after the oral dose and from 40.7 h to 21.6 hours after the rectal dose under the a.c. treatment. It is inferred from these results that piroxicam is subject to enteral circulation. A.c. appears useful as an antidote in acute intoxications from piroxicam.