Pharmacology Basics

Pharmacology is the study of drugs and their effects on living organisms. It encompasses how drugs are absorbed, distributed, metabolized, and excreted (pharmacokinetics), as well as how they produce their effects (pharmacodynamics).

Pharmacokinetics (PK): what the body does to the drug (ADME: absorption, distribution, metabolism, excretion). Pharmacodynamics (PD): what the drug does to the body (mechanism of action, effects, dose-response relationships).

ADME stands for Absorption, Distribution, Metabolism, and Excretion — the four processes that determine how a drug moves through the body. Together they determine the drug's concentration at the site of action and its duration of effect.

Drug absorption is the movement of a drug from its site of administration into the bloodstream. Factors affecting absorption: route of administration, drug solubility, pH, blood flow to absorption site, surface area, and drug formulation.

Bioavailability is the fraction of an administered drug that reaches the systemic circulation in active form. IV drugs have 100% bioavailability. Oral drugs have lower bioavailability due to incomplete absorption and first-pass metabolism.

The first-pass effect is the metabolism of an orally administered drug by the liver before it reaches systemic circulation. The drug is absorbed from the GI tract into the portal vein, passes through the liver, and may be significantly metabolized, reducing bioavailability.

Routes include: oral (PO), intravenous (IV), intramuscular (IM), subcutaneous (SC), sublingual (SL), transdermal, inhalation, rectal, topical, and intrathecal. Each route has different absorption rates, bioavailability, and clinical applications.

IV administration delivers drugs directly into the bloodstream, providing 100% bioavailability and immediate effect. It is used for emergencies, drugs with poor oral absorption, precise dosing, and when rapid onset is needed. Disadvantages: infection risk, cannot be easily reversed.

Drug distribution is the process by which a drug moves from the bloodstream to body tissues. Factors: blood flow, drug lipophilicity, protein binding, tissue permeability, and the blood-brain barrier. Highly perfused organs (heart, liver, kidneys) receive drugs first.

Protein binding is when drugs bind to plasma proteins (mainly albumin) in the blood. Only the unbound (free) fraction is pharmacologically active. Highly protein-bound drugs (>90%) have a smaller free fraction. Drug interactions can occur when two drugs compete for binding sites.

Volume of distribution is a theoretical volume that relates the total amount of drug in the body to its plasma concentration: Vd = dose/plasma concentration. A large Vd indicates the drug distributes extensively into tissues; a small Vd suggests it stays mostly in plasma.

Drug metabolism is the chemical modification of drugs by the body, primarily in the liver. It typically converts lipophilic drugs into more water-soluble metabolites for excretion. Metabolism occurs in two phases: Phase I (oxidation, reduction, hydrolysis) and Phase II (conjugation).