Body and drug basics
First-pass metabolism
Last updated
Anything absorbed from the gut travels to the liver before it reaches the rest of the body, and the liver breaks down a share of it on that first trip through. That share never reaches general circulation at all. It is the main reason a swallowed substance can be absorbed well and still barely register in the blood.
Why it matters here
It explains why several routes exist in the first place: under the tongue, through the nose and by injection all bypass this first trip, which is otherwise unavoidable for anything swallowed. It is one of the largest single reasons the oral and injected figures for the same substance differ so widely.
What it is not
Not the same as poor absorption, though the two are constantly confused because they produce the same disappointing result. Absorption can be excellent and first-pass loss still severe; they are separate steps with separate causes. For most peptides something earlier happens anyway, since digestion takes the chain apart before the liver is ever reached.
Also called
- first-pass metabolism
- first pass metabolism
- first-pass effect
- first pass effect
- first pass
- hepatic first pass
- presystemic metabolism
Also called the first-pass effect or presystemic metabolism, presystemic meaning before the substance reaches the general circulation.
Compounds carrying this term
None on file yet. This term is in the controlled vocabulary but no compound record currently uses it — which is a statement about our coverage, not about the term.