First-Pass Metabolism

First-pass metabolism is the loss of drug by metabolism before it reaches the systemic circulation, occurring chiefly in the gut wall and liver as orally absorbed drug is carried by the portal vein through the liver. Because this presystemic loss can be large, it is a principal reason why an oral dose may yield far lower systemic exposure than the same dose given intravenously.

Definition

First-pass metabolism (the first-pass or presystemic effect) is the metabolism of an absorbed drug — principally in the intestinal wall and liver — that occurs before the drug reaches the systemic circulation, reducing the fraction of the dose that becomes systemically available.

Scope

This topic explains the first-pass (presystemic) effect: where it happens, why it predominantly affects orally administered drugs, and how it links route of administration to bioavailability. It treats the effect as a pharmacokinetic mechanism and offers no dosing or therapeutic guidance.

Key concepts

• Presystemic (first-pass) elimination
• Portal circulation and hepatic transit
• Gut-wall and hepatic metabolism
• Extraction ratio
• High first-pass drugs and low oral bioavailability
• Routes that bypass the first pass

Mechanisms

After absorption from the gastrointestinal tract, drug enters the hepatic portal vein and passes through the liver before reaching the general circulation; metabolising enzymes in the intestinal wall and liver can remove a substantial fraction during this transit. The proportion removed is summarised by the extraction ratio, and drugs with a high extraction ratio show low and variable oral bioavailability. Routes that drain into the systemic rather than the portal circulation — such as sublingual, transdermal, or intravenous — avoid or reduce this first pass, as Pond and Tozer set out.

Clinical relevance

First-pass metabolism explains why some drugs have low oral bioavailability and why exposure can vary between people and routes; recognising it is part of interpreting pharmacokinetic differences. This entry describes the mechanism for educational and appraisal purposes and is not a basis for individual dosing or treatment choices.

Evidence & guidelines

Pond and Tozer's 1984 review remains the standard synthesis of first-pass concepts, and general pharmacokinetic texts such as Rowland and Tozer formalise the extraction ratio and its effect on bioavailability. The Biopharmaceutics Classification System of Amidon and colleagues situates presystemic loss within the broader determinants of oral exposure.

History

The first-pass effect was recognised as clinical pharmacokinetics matured in the 1970s and consolidated by Pond and Tozer's 1984 review, which framed the basic concepts and clinical consequences of presystemic elimination. Later work distinguished the gut-wall and hepatic contributions to this loss, refining how oral bioavailability is understood.

Key figures

• Susan M. Pond
• Thomas N. Tozer
• Malcolm Rowland
• Gordon L. Amidon

Seminal works

• pond-tozer-1984

Frequently asked questions

Why does first-pass metabolism mostly affect oral drugs?

Drug absorbed from the gut is carried by the portal vein through the liver before reaching the general circulation, so it can be metabolised in the gut wall and liver first; routes that enter the systemic circulation directly largely avoid this.

How does first-pass metabolism relate to bioavailability?

It reduces the fraction of an absorbed dose that reaches the systemic circulation, so a high first-pass effect produces low oral bioavailability even when the drug is well absorbed from the gut.