The gastrointestinal absorption of methadone in the rat.

The absorption of dl-methadone from the gastrointestinal tract of the Sprague-Dawley rat was examined by the in vivo segment technique. Duodenal absorption, measured as a function of time and dose, followed first-order kinetics with a half-life of 15.6 min. Absorption was not influenced by prior or concomitant administration of a variety of drugs. Absorption from other regions of the intestine was similar to that from the duodenum; in contrast, absorption from the stomach was markedly slower. Gastric absorption was increased by alkalinization of stomach contents but was still considerably slower than from the duodenum. Gastric emptying of methadone appears to be the rate-limiting step in the overall gastrointestinal absorption of the drug, since the rate of emptying following intubation of the drug into the stomach was also considerably slower than the rate of duodenal absorption.