First-Pass Metabolism: Why Pills Lose Drug Early
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[Glossary · How doses act in the body]

First-pass metabolism

Also called first-pass effect, presystemic metabolism, first pass · Reviewed

Definition: First-pass metabolism, or the first-pass effect, is when a drug is broken down at one site before it reaches the general circulation, lowering the amount that gets through. The liver is the best-known site, but StatPearls notes it can also happen in the gut, lungs, blood vessels and other active tissues.

What changes it

StatPearls says the effect varies from person to person and can be changed by blood protein levels, how fast the gut moves, and enzyme activity. When it is strong, a drug may need a different route or form to get around it.

First pass is one reason bioavailability by mouth is often lower than by injection. The FDA guidance notes that blood level profiles reflect both release of the drug and presystemic changes to it, which is why an IV reference dose is used to measure absolute bioavailability.

Worked example: one molecule, two routes

Semaglutide by injection vs by tablet, Wegovy label

  1. Injection under the skin: absolute bioavailability 89%.
  2. Tablet: absolute bioavailability about 1% to 2%, with absorption mostly in the stomach.
  3. Out of 100 mg swallowed, 1% to 2% is 1 to 2 mg reaching the blood. Out of 1 mg injected, 89% is 0.89 mg.
  4. The label does not split the oral loss into causes, so not all of it is first-pass metabolism. Poor absorption through the gut wall counts too.

This is why the tablet and injection have very different label doses. Neither number transfers to the other route.

Why it matters when you track doses

If you log a drug that comes in more than one form, log the route with each dose. As the Wegovy numbers show, a milligram by mouth and a milligram by injection are not the same exposure.

Read more on Dose Track: Wegovy pill guide · Wegovy guide

General education drawn from labels and published studies, not personal medical advice. Your prescriber, pharmacist and the label that came with your product are the final word.
BioavailabilityBioavailability is how much of a dose, and how fast, gets absorbed into the body where it can act. The FDA… Subcutaneous injection (SubQ)A subcutaneous injection (SubQ) puts medicine into the layer of fat just under the skin, above the muscle.… PeptideA peptide is a short chain of amino acids, the same building blocks that make up proteins. US rules draw… ClearanceClearance is the volume of blood plasma the body clears of a drug per unit of time, so it is written as…

Frequently asked questions

Do injections avoid first-pass metabolism?+
StatPearls says a strong first-pass effect may call for a different route or form. A shot under the skin does not pass through the gut first. The drug is still removed later, which is what clearance measures.
Why is oral semaglutide bioavailability so low?+
The Wegovy label puts it at about 1% to 2% for the tablet, against 89% for the injection. The label does not break that gap down by cause.

Sources

  1. Herman TF, Santos C. First-Pass Effect. StatPearls 2023. PMID 31869143
  2. FDA guidance for industry: Bioavailability Studies Submitted in NDAs or INDs, General Considerations (April 2022)
  3. Wegovy (semaglutide) US prescribing information, sections 2 and 12.3, DailyMed

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