3.2 : Drug Absorption: Overview

The process of drug absorption signifies the transition of a drug from its site of administration into the plasma. This process is influenced by various factors, including the route of administration, the anatomy of the absorption site, the mechanism of absorption, gut motility, and the drug's physicochemical properties.

When drugs are injected intravenously, they directly enter the systemic circulation. Alternatively, orally administered drugs navigate through the gastrointestinal (GI) tract. Here, the intestinal villi, which are rich in blood supply, offer a vast surface area for drug absorption.

Drugs typically diffuse passively or actively use transporters to traverse the intestinal cell membrane, thereby gaining entry into the bloodstream. Some drugs actively use specific transporters, while others may use facilitated diffusion, which is still passive. However, alterations in GI motility, such as gastric stasis or rapid gut content movement, can adversely affect drug absorption. Also, the pH of the absorption site is an important factor that influences drug ionization and, consequently, absorption.

Solid drugs' rate of dissolution limits their absorption capabilities. Drugs that are nonionized and lipophilic are favored for absorption, whereas less lipophilic drugs like strong acids or bases experience poor absorption in the gut.

In practice, only a fraction of the total administered dose, referred to as bioavailability, successfully reaches the bloodstream as an intact drug. This highlights the complexities and intricacies involved in the drug absorption process.