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en.wikipedia.org/wiki/First_pass_effect

This first pass through the liver thus greatly reduces the bioavailability of the drug. 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.

www.sciencedirect.com/.../first-pass-effect

First Pass Effect. A first-pass effect is defined as the rapid uptake and metabolism of an agent into inactive compounds by the liver, immediately after enteric absorption and before it reaches the systemic circulation.

medical-dictionary.thefreedictionary.com/first-pass+effect

first-pass effect: the intestinal and hepatic degradation or alteration of a drug or substance taken by mouth, after absorption, removing some of the active substance from the blood before it enters the general circulation. Synonym(s): first-pass effect

socratic.org/questions/what-is-meant-by-the-first-pass-effect-of-drugs

first-pass effect is a process in which a drug administered by mouth is absorbed from the gastrointestinal tract and transported via the portal vein to the liver, where it is metabolized. As a result, in cases of some drugs, only a small proportion of the active drug reaches the systemic circulation and its intended target tissue.

quizlet.com/70075057/first-pass-effect-flash-cards

Because the liver is the primary organ of metabolism, the consequence of first-pass metabolism is the break down of significant amounts of a drug before it can reach the systemic circulation, so those drugs never reach the site of action!

study.com/academy/lesson/the-first-pass-effect-in-pharmacology.html

The First Pass Effect. When you take a medication by mouth, it doesn't just magically get into your body and start doing its thing. It actually has to go through a whole host of organs and a big ...

www.ncbi.nlm.nih.gov/pubmed/6362950

When several sites of first-pass metabolism are in series, the bioavailability is the product of the fractions of drug entering the tissue that escape loss at each site. The extent of first-pass metabolism in the liver and intestinal wall depends on a number of physiological factors.

www.labce.com/spg1094059_first_pass_hepatic_metabolism.aspx

First Pass Hepatic Metabolism Phase I metabolic reactions can occur during the absorptive phase in the gut wall or liver before reaching the blood stream. This results in a reduction in the concentration of the drug before it reaches the circulation.

www.researchgate.net/post/Can_someone_provide_a_simple_explanation_for_the_1st...

With most psychoactive substances, first pass liver metabolism can make a very significant difference in the amount of the drug that ends up reaching the brain and other organs.