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Bioavailability and First-Pass Effect of Drugs

Definition of Bioavailability

Bioavailability describes the (unchanged) fraction of a drug, which appears within the blood circulation after administration. By definition, intravenous administration correlates with 100% bioavailability. Various mechanisms such as incomplete absorption and first-pass effect reduce the bioavailability of orally administered drugs.

Mechanisms of a Low Bioavailability of Oral Medication

Incomplete Solution

Only dissolved drugs can be absorbed by the gastrointestinal tract.

Incomplete Absorption

The absorption of drugs through the intestinal wall is time dependent and is affected by the velocity of gastrointestinal passage. Diseases such as diarrhea, vomiting, short bowel syndrome or gastrointestinal fistulas are additional mechanisms of incomplete absorption.

First-Pass Effect in the Intestine and Liver

After gastrointestinal absorption, drugs may be metabolized and inactivated in the intestinal wall (e.g., catecholamines). Subsequently, the drug enters the portal circulation to the liver. Depending on the liver function and enzyme activity, further inactivation of the drug may occur (e.g., morphine). The first pass effect can be minimized by rectal, sublingual, transdermal, subcutaneous, intramuscular or nasal administration.

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  Deutsche Version: Bioverfügbarkeit