This document discusses drug absorption, which is defined as the movement of an unchanged drug from its site of administration into systemic circulation. Most drugs are administered orally, so they must be absorbed through the gastrointestinal tract before exerting pharmacological effects. Both the rate and extent of drug absorption are important for achieving therapeutic drug concentrations. Drugs can be administered via enteral routes like oral or rectal, parenteral routes like intravenous or intramuscular, or topical routes like skin or eyes.