First pass effect
1173539
217018735
2008-06-04T07:20:43Z
Sween64
2841795
added suppository
The '''first-pass effect''' (also known as '''first-pass metabolism''' or '''presystemic metabolism''') is a phenomenon of [[drug metabolism]] whereby the [[concentration]] of a [[medication|drug]] is greatly reduced before it reaches the systemic circulation.
After a drug is swallowed, it is absorbed by the [[digestive system]] and enters the [[hepatic portal system]]. It is carried through the [[Hepatic portal vein|portal vein]] into the [[liver]] before it reaches the rest of the body. The liver [[metabolism|metabolizes]] many drugs, sometimes to such an extent that only a small amount of active drug emerges from the liver to the rest of the [[circulatory system]]. This ''first pass'' through the liver thus greatly reduces the [[bioavailability]] of the drug. Alternative [[route of administration|routes of administration]] like [[suppository]], [[intravenous]], [[intramuscular]], and [[sublingual]] avoid the first-pass effect because they allow drugs to be absorbed directly into the [[systemic circulation]].
The four primary systems that affect the first pass effect of a drug are the enzymes of the [[Gastrointestinal tract|gastrointestinal]] [[Lumen (anatomy)|lumen]], gut wall enzymes, bacterial enzymes, and hepatic enzymes.
==See also==
* [[First dose effect]], a similarly-named but basically unrelated topic
* [[ADME]], an acronym in [[pharmacokinetics]] and [[pharmacology]] standing for absorption, distribution, metabolism, and excretion
* [[Partition coefficient]]
* [[Biopharmaceutics Classification System]]
* [[Drug]]
[[Category:Pharmacokinetics]]
[[Category:Medicinal chemistry]]
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