A drug is fundamentally a chemical agent that interacts with biological systems to induce a change in physiological function. These substances exert their effects by binding to specific macromolecules, such as receptors, enzymes, or ion channels, on target cells. The mechanism of action determines whether the substance acts as an agonist, mimicking the effect of a natural signaling molecule, or as an antagonist, thereby blocking the natural binding site and preventing the signal from propagating. Pharmacological classification relies heavily on understanding these molecular interactions and the resulting effects on biological pathways, determining the therapeutic window and potential side effects based on the substance's affinity and selectivity for its intended targets.