- First pass effect
The first-pass effect (also known as first-pass metabolism or presystemic metabolism) is a phenomenon of
drug metabolism whereby theconcentration of a drug is greatly reduced before it reaches the systemic circulation.After a drug is swallowed, it is absorbed by the
digestive system and enters thehepatic portal system . It is carried through the portal vein into theliver before it reaches the rest of the body. The liver metabolizes many drugs, sometimes to such an extent that only a small amount of active drug emerges from the liver to the rest of thecirculatory system . This "first pass" through the liver thus greatly reduces thebioavailability of the drug. Alternative routes of administration likesuppository ,intravenous ,intramuscular , andsublingual avoid the first-pass effect because they allow drugs to be absorbed directly into thesystemic circulation .The four primary systems that affect the first pass effect of a drug are the enzymes of the gastrointestinal lumen, gut wall enzymes, bacterial enzymes, and hepatic enzymes.
ee also
*
First dose effect , a similarly-named but basically unrelated topic
*ADME , an acronym inpharmacokinetics andpharmacology standing for absorption, distribution, metabolism, and excretion
*Partition coefficient
*Biopharmaceutics Classification System
*Drug
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