First-pass effect

The first-pass effect is metabolism of an orally administered drug in the intestinal wall or liver before it reaches systemic circulation, reducing the amount of unchanged drug available.

Portal vein: A large vein that carries blood from the digestive organs and spleen to the liver. Absorbed oral drugs commonly reach the liver through this vessel before entering systemic circulation.
Oral administration: The delivery of a drug by swallowing it for absorption through the gastrointestinal tract. This route exposes many drugs to gut-wall and hepatic metabolism before systemic entry.
First-pass metabolism: Drug metabolism in the intestinal wall or liver before a substance first enters systemic circulation. This phrase names the metabolic process underlying the first-pass effect.
Pharmacokinetics: The study of how the body absorbs, distributes, metabolizes, and excretes drugs. The first-pass effect is one determinant of a drug’s concentration over time.
Hepatic metabolism: The chemical transformation of substances by enzymes in the liver. Liver enzymes can metabolize a drug during its first passage from the gut.
Sublingual administration: Drug delivery by absorption beneath the tongue into systemic venous blood. This route can avoid initial passage through the liver.
Drug clearance: The volume of plasma from which a drug is completely removed per unit time. Clearance describes drug removal after systemic entry, unlike presystemic first-pass loss.
Drug metabolism: The enzymatic chemical alteration of drugs in the body. First-pass metabolism is a location- and timing-specific form of drug metabolism.
Intestinal metabolism: The chemical transformation of substances by enzymes in the intestinal wall. Gut-wall enzymes can remove drug before it reaches the portal vein.
Intravenous therapy: Administration of a substance directly into a vein. Intravenous dosing delivers drug directly into systemic circulation, bypassing first-pass loss.

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