Enteral (administration)

Enteral administration refers to administration of a drug by a route that then requires the drug to be absorbed through the walls of the GI tract (stomach, intestines) and pass through the liver in the portal circulation before reaching the systemic circulation. Enteral routes of administration include PO and PR (rectal), although a suppository that is not pushed in much beyond the anal sphincter may be absorbed, at least in part, into blood vessels around the anus that do not drain into the portal circulation.

Since drugs administered by an enteral route must pass through the liver, they may undergo first pass metabolism. This process results in loss of a significant portion of a dose of a high hepatic extraction ratio drug before it ever reaches the systemic circulation, which is why these drugs typically have low oral bioavailability.

Some drugs administered by an enteral route may undergo a degree of lymphatic drug absorption, which obviates the need for initial passage through the liver and which can increase oral bioavailability to a value greater than might be predicted based upon the hepatic extraction ratio for that drug.