A drug is a chemical substance introduced into the body that alters the function of a cell, organ, or system. The drug’s journey—moving through the body, binding to its target, and being eliminated—is a carefully orchestrated sequence of events. For a drug to be effective, it must reach its intended site of action at a concentration high enough to produce a therapeutic effect without causing widespread harm. This multi-step process, from administration until complete removal, determines its overall success as a treatment.
Getting the Drug into the System
The initial stage of a drug’s journey is absorption: moving from the site of administration into the bloodstream. The route of administration (oral, injection, or topical) influences the speed and extent of absorption. Orally administered drugs must survive the harsh environment of the digestive tract and pass through the intestinal wall to enter circulation.
Drugs administered directly into a vein bypass absorption entirely, achieving 100% bioavailability—the fraction reaching systemic circulation unchanged. Once absorbed, the drug is distributed throughout the body via the circulatory system to various tissues and organs. Distribution depends on blood flow; highly perfused organs like the brain, heart, and liver receive the drug more quickly than fat or bone.
Specialized biological structures, such as the blood-brain barrier, limit a drug’s movement into certain areas, protecting the central nervous system. During distribution, many drugs temporarily bind to plasma proteins, such as albumin, in the bloodstream. Only the “free,” unbound portion of the drug can leave the circulation, interact with its molecular target, and produce a therapeutic effect.
Binding to plasma proteins acts as a temporary reservoir, regulating the concentration of active drug available to tissues. A drug’s chemical properties, particularly its lipid solubility, influence its ability to cross cell membranes and distribute effectively. The drug must balance water and fat solubility to be absorbed, travel through the blood, and cross the fatty membranes of target cells.
Targeting Cells and Triggering Effects
Once distributed, the drug interacts with the body on a molecular level, a process known as pharmacodynamics. Drugs exert their effects by binding to specific molecular targets, which are typically large protein molecules on the surface or inside of cells. These targets include receptors, enzymes, ion channels, and transporters, designed to interact with the body’s natural signaling molecules.
The interaction is often described by the “lock and key” model, where the drug molecule (the key) fits precisely into the binding site of the target protein (the lock). This specific fit ensures the drug primarily affects the intended biological pathway, minimizing side effects. Binding to a receptor triggers a change in the cell’s activity, leading to the desired therapeutic outcome.
Drugs that activate the target protein and mimic the action of natural signaling molecules are called agonists. Agonists cause a cellular response, similar to turning a switch “on.” Other drugs, known as antagonists, bind to the same target site but do not activate it; instead, they occupy the site and physically block the natural signaling molecule from binding.
Antagonists act as blockers, preventing the natural signal from reaching the receptor and dampening an excessive biological response. The intensity of the drug’s effect is directly related to the dose given, following the dose-response relationship. Increasing the drug concentration leads to a greater number of occupied target proteins, resulting in a stronger effect until saturation occurs.
Breaking Down and Removing Drugs
The final stage involves metabolism and excretion, which eliminate the drug from the body. Metabolism, or biotransformation, primarily occurs in the liver. Its goal is to chemically alter the fat-soluble drug molecule into a more water-soluble form, making it easier for the kidneys to filter and remove.
The Cytochrome P450 (CYP450) enzyme system is responsible for metabolizing 70% to 80% of all drugs in clinical use. These enzymes perform chemical reactions that add or expose polar groups (like hydroxyl or carboxyl groups), increasing water solubility. Genetic differences affect how quickly these CYP450 enzymes work, leading to variability in drug response among individuals.
The drug’s half-life, determined by metabolism and excretion, represents the time required for the drug concentration in the body to be reduced by half. This measurement guides how frequently a drug must be dosed to maintain a concentration within the therapeutic range.
Excretion is primarily handled by the kidneys, which filter the blood and eliminate the drug metabolites via the urine. Other routes include the gut (feces), breath, or sweat. Impaired function of the liver or kidneys slows this process, prolonging the drug’s half-life and potentially leading to a dangerous buildup in the body.