Bioavailability: Overview

Bioavailability indicates the fraction of the total administered dose that reaches the systemic circulation as an intact drug.

Different administration routes result in varying bioavailability. For example, the drug directly enters the bloodstream through intravenous injection, providing 100% bioavailability.

However, orally administered drugs pass through the GI tract, get absorbed, and undergo metabolism in the liver, resulting in a lesser dose of the drug reaching the systemic circulation.

An oral drug’s bioavailability depends on enzyme activity in the gut wall or liver, gastric pH, and intestinal motility. It is the ratio of the drug quantity reaching the systemic circulation to the drug quantity administered.

Some drugs, like lidocaine, undergo extensive first-pass metabolism, significantly reducing their bioavailability and ruling out their oral usage.

Bioavailability studies for a new or existing drug indicate their therapeutic utility, absorption efficiency, suitable formulation, and effective administration route.