Pharmacokinetics of fentanyl as a possible explanation for recurrence of respiratory depression.

The pharmacokinetics of fentanyl are complicated by an additional increase in plasma concentration during the elimination phase of the drug. We have confirmed that fentanyl is excreted in the gastric juice and reabsorbed from the alkaline medium of the small intestine. In addition, the stomach wall in rats has an important storage capacity for fentanyl. A maximum of about 20% of the dose was found in the stomach wall after i.v. injection. In man the resected part of the stomach contained 16% of the dose, 10 min after injection. These observations could be important in explaining the occurrence of respiratory depression in the period after operation.