First-pass metabolism

The first pass effect (or first pass metabolism) is a phenomenon of drug metabolism.

After a drug is swallowed, it is absorbed by the digestive system and enters the portal circulation. The absorbed drug is carried through the portal vein into the liver.

The liver is responsible for metabolizing many drugs. Some drugs are so extensively metabolized by the liver that only a small amount of unchanged drug may enters the systemic circulation, so the bioavailability of the drug is reduced.

Alternative routes of administration (e.g. intravenous, intramuscular) avoid the first pass effect.

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