Mechanism of Drug Absorption

Absorption: Mechanisms of drug absorption through the GIT

Introduction

Drug absorption is one of the most important processes in biopharmaceutics because it determines how a drug moves from its site of administration into the systemic circulation. For orally administered medicines, the gastrointestinal tract (GIT) is the major route through which most drugs are absorbed. The efficiency of absorption has a direct influence on the onset, intensity and duration of drug action and ultimately affects the therapeutic response.

After oral administration, a drug must undergo several steps before reaching the systemic circulation. These include disintegration of the dosage form, dissolution of the drug, and movement of the dissolved drug across the gastrointestinal membrane. The extent and rate of absorption depend on several factors, including the physicochemical properties of the drug, dosage-form characteristics, and physiological conditions of the GIT.

Drugs can cross the gastrointestinal membrane through several mechanisms. The major mechanism is passive diffusion, in which drug molecules move across the biological membrane from a region of higher concentration to a region of lower concentration without requiring cellular energy. Other mechanisms include facilitated diffusion, active transport, and endocytosis. Some drugs may also pass through the spaces between cells by paracellular transport. The relative importance of each mechanism depends on the molecular size, lipid solubility, ionisation, concentration gradient, and structural characteristics of the drug.

Understanding the mechanisms of drug absorption through the GIT is essential for predicting the bioavailability of orally administered drugs and for designing effective pharmaceutical dosage forms. Factors such as GIT pH, gastric emptying, intestinal motility, gastrointestinal blood flow, surface area, drug solubility, and membrane permeability can significantly influence drug absorption.

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