The process of drug absorption involves the passage of a drug from its administration site into the plasma. It is influenced by factors like administration route, anatomy of absorption site, absorption mechanism, gut motility, and the drug's physicochemical properties. Intravenously injected drugs directly enter the systemic circulation. Oral drugs travel through the GI tract, where intestinal villi, rich in blood supply provide a large surface area for their absorption.

Drugs passively diffuse or use transporters to move across the intestinal cell membrane, ultimately entering the bloodstream.

Changes in GI motility, such as gastric stasis or rapid gut content movement, can impair drug absorption.

For solid drugs, the rate of dissolution limits their absorption. Nonionized and lipophilic drugs are favored, whereas less lipophilic drugs like strong acids or bases are poorly absorbed in the gut.

In reality, only a fraction of the total administered dose called the bioavailability, reaches the bloodstream as an intact drug.