M06.01.02 Drug Absorption

Overview: Drug absorption is a critical process that determines the extent and rate at which a drug enters the bloodstream after administration. It involves the movement of a drug from its site of administration into the systemic circulation, where it can exert its therapeutic effects.

Routes of Administration:

• Drugs can be administered through various routes, including oral (by mouth), intravenous (IV), intramuscular (IM), subcutaneous (SC), transdermal (through the skin), inhalation, and rectal routes.
• Each route has unique characteristics that influence drug absorption, such as the presence of barriers (e.g., gastrointestinal tract mucosa, skin), blood flow, and drug formulation.

Factors Influencing Drug Absorption:

• Physicochemical Properties: The chemical structure of a drug influences its solubility, lipophilicity, and ionization state, which affect its ability to cross biological membranes.
• Formulation: Drug formulations (e.g., tablets, capsules, solutions) can impact dissolution and disintegration rates, thereby influencing absorption.
• pH Environment: The pH of the gastrointestinal tract affects drug ionization and solubility, influencing absorption. For example, weak acids are better absorbed in acidic environments, while weak bases are better absorbed in alkaline environments.
• Blood Flow: Blood perfusion at the site of administration affects drug absorption rates. Areas with high blood flow, such as the intestines and muscles, typically exhibit faster absorption.
• Drug-Drug Interactions: Co-administration of drugs can influence absorption through various mechanisms, such as altering gastrointestinal motility, competing for transporters, or affecting drug metabolism.
• First-Pass Effect: Drugs absorbed orally pass through the liver before reaching systemic circulation, where they may undergo extensive metabolism (first-pass metabolism), reducing their bioavailability.

Bioavailability and Pharmacokinetics:

• Bioavailability refers to the fraction of the administered dose that reaches systemic circulation unchanged and is available to exert pharmacological effects.
• Understanding drug absorption is crucial for predicting drug concentrations in the bloodstream over time (pharmacokinetics) and determining appropriate dosing regimens.

Clinical Implications:

• Variability in drug absorption among individuals can lead to differences in therapeutic response and adverse effects.
• Drug absorption is particularly important in clinical practice when selecting the most appropriate route of administration and dosage form for a given patient and drug.

In summary, drug absorption is a complex process influenced by various factors, including physicochemical properties, formulation, pH environment, and blood flow. Understanding these factors is essential for optimizing drug therapy and ensuring therapeutic efficacy and safety.