Pharmacodynamics

Pharmacodynamics is the study of how drugs produce their effects at the cellular level. Drugs act primarily by binding to receptors (proteins on or inside cells). An agonist binds a receptor and activates it, mimicking the body's own chemical signals — albuterol activates beta-2 receptors to relax bronchial smooth muscle. An antagonist binds the receptor but blocks activation — propranolol blocks beta receptors, preventing the heart rate increase that epinephrine would cause. Partial agonists (like buprenorphine) activate receptors but produce a submaximal response, which creates a ceiling effect. Two critical measurable concepts: potency refers to how much drug is needed to produce an effect (lower dose = higher potency), while efficacy refers to the maximum effect a drug can achieve regardless of dose. A drug can be highly potent but have low efficacy, or vice versa. Dose-response relationships follow a predictable curve — the therapeutic effect increases with dose until a plateau, after which more drug adds toxicity without benefit. The therapeutic index (TI) compares the toxic dose to the therapeutic dose; a narrow TI (like digoxin, lithium, warfarin) means the margin between help and harm is slim.