What does the term "bioavailability" refer to in pharmacology?

Bioavailability is a critical concept in pharmacology that defines the proportion of a drug that enters the systemic circulation when it is introduced into the body. It essentially measures how much of the drug is available to exert its effects after administration. This can be influenced by various factors, including the drug's formulation, route of administration, and the first-pass metabolism that occurs in the liver.

When a drug is administered intravenously, its bioavailability is typically considered to be 100%, as it directly enters circulation. However, for other routes, such as oral administration, the bioavailability can be significantly less due to factors such as absorption in the gastrointestinal tract and metabolism before the drug reaches the systemic circulation. Understanding bioavailability is essential for determining dosing regimens and understanding the drug's therapeutic effects in the body.