Oral (PO) administration

Oral administration (abbreviated as PO, from “per os”) refers to the oral administration of a tablet, capsule or liquid to a patient. This is an enteral route of administration. After the drug is swallowed, it is absorbed across the walls of the stomach and/or intestines into blood vessels that drain to the portal vein that enters the liver. The drug must then pass through the liver before it reaches the systemic circulation and distributes to tissues.

If the drug is rapidly metabolised by hepatic enzymes (i.e if the drug is a high hepatic extraction ratio drug), a significant portion of the administered dose can be lost before it ever reaches the systemic circulation. This is referred to as first pass metabolism – the oral bioavailability of the drug is lower for drugs that undergo extensive first pass metabolism. Several very lipid-soluble drugs can, in part, reach the systemic circulation following lymphatic drug absorption, with the portion of the drug absorbed in this way escaping first-pass metabolism.