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en.wikipedia.org/wiki/First_pass_effect

The first pass effect (also known as first-pass metabolism or presystemic metabolism) 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 drug lost during the process of absorption which is generally related to the liver and gut wall.

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

A first-pass effect is defined as a low systemic availability of the drug as a result of significant metabolism. Although a first-pass effect can occur in a variety of tissues, including the intestines Doherty and Pang (1997) and uterus De Ziegler et al (1997), it is most often observed with the liver.

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

first-pass metabolism 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 first-pass me·tab·o·lism , first-pass effect (fĭrst-pas mĕ-tab'ŏ-lizm, e-fekt') The ...

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

First-pass elimination takes place when a drug is metabolised between its site of administration and the site of sampling for measurement of drug concentration. Clinically, first-pass metabolism is important when the fraction of the dose administered that escapes metabolism is small and variable ...

www.nottingham.ac.uk/nmp/sonet/rlos/bioproc/metabolism/01.html

Understanding First Pass Metabolism : Drug Absorption. The drug is absorbed from the GI tract and passes via the portal vein into the liver where some drugs are metabolised. Sometimes the result of first pass metabolism means that only a proportion of the drug reaches the circulation. First pass metabolism can occur in the gut and the liver.

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.

www.magonlinelibrary.com/doi/abs/10.12968/npre.2017.15.6.303

In this article in the series of ‘bite sized’ pharmacology, we will look at the concept of first pass metabolism. All drugs given by the oral route undergo a degree of first pass metabolism either in the gut or the liver, with some drugs being destroyed before they reach the systemic circulation.

www.labce.com/spg1094059_first_pass_hepatic_metabolism.aspx

First pass metabolism determines what fraction of an oral dose will reach the circulation - the bioavailable fraction. Intravenous drugs don’t experience this first pass effect and are, by definition, 100% bioavailable. Drugs administered orally or inhaled demonstrate less than 100% bioavailability.

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 ...