First pass effect

First pass effect

The First pass effect (pronounced: /fɜːrst pæs ɪˈfɛkt/), also known as presystemic metabolism or liver first pass, is a phenomenon of drug metabolism whereby the concentration of a drug is greatly reduced before it reaches the systemic circulation. It is the fraction of lost drug during the process of absorption which is generally related to the liver and gut wall.

Etymology

The term "First pass effect" is derived from the process where the drug is first metabolized in the liver before it passes into the general circulation. The term "presystemic metabolism" refers to the metabolic activities occurring before the drug enters the systemic circulation.

Mechanism

The first pass effect is a phenomenon of drug metabolism whereby the concentration of a drug, specifically when administered orally, is greatly reduced before it reaches the systemic circulation. The 'first pass' refers to the initial passage of the drug from the gut to the liver through the portal vein, following oral administration.

The first pass effect can result in greatly reduced bioavailability of the drug, which is the degree to which the drug becomes available to the target tissue after administration. Drugs that undergo a significant first pass effect must be administered in higher doses than drugs that do not in order to achieve therapeutic effect.

Related Terms

• Bioavailability
• Drug metabolism
• Hepatic portal system
• Liver
• Pharmacokinetics

See Also

• Pharmacology
• Pharmacodynamics
• Drug interaction

External links

• Medical encyclopedia article on First pass effect
• Wikipedia's article - First pass effect

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