Pharmacodynamics describes how a drug produces its effects at the molecular, cellular, and whole-organism level. The dose-response curve plots the drug dose on the x-axis against the magnitude of the response on the y-axis and allows two important properties to be visualized: potency, reflected by the position of the curve along the x-axis and indicating how much drug is needed to produce an effect, and efficacy, reflected by the maximum height of the curve and indicating the largest effect the drug can produce regardless of dose. The ED50 is the dose that produces 50 percent of the maximum effect. Although a more potent drug is sometimes assumed to be clinically superior, efficacy is generally considered more important than potency because it represents the maximum therapeutic benefit that can be achieved.
Most drugs act by binding to specific receptors, and the nature of this binding determines the type of drug-receptor interaction. An agonist binds to a receptor and activates it to produce a biological response, with full agonists producing a maximal response and partial agonists producing only a submaximal response even when all available receptors are occupied. An antagonist binds to a receptor but does not activate it, instead blocking the action of agonists; competitive antagonists bind to the same site as the agonist and their effect can be overcome by increasing the agonist concentration, whereas non-competitive antagonists bind elsewhere and cannot be overcome in this way. A partial agonist can also act as an antagonist in the presence of a full agonist by competing for the same receptors, as seen with buprenorphine at opioid receptors.
The therapeutic index (TI) quantifies the safety margin of a drug and is defined as the ratio of the toxic dose to the effective dose, often expressed as TI = TD50 / ED50, where TD50 is the dose that produces toxicity in 50 percent of subjects and ED50 is the dose that produces the desired effect in 50 percent. A high therapeutic index indicates a wide margin between effective and toxic doses, while a narrow therapeutic index, as is found for warfarin, lithium, digoxin, and phenytoin, means that small changes in dose or blood concentration can produce toxicity, so these drugs require careful dose titration and often therapeutic drug monitoring.