Principles of pharmacokinetics and pharmacodynamics

Pharmacokinetics is the scientific discipline that studies the time course of absorption, distribution, metabolism, and excretion of drugs. Parameters describing a drug's pharmacokinetics are determined with a number of different methods, including compartmental and noncompartmental methods. Two primary parameters that are determined include clearance and volume of distribution. Clearance is a parameter that measures the elimination of drug from the body, whereas volume of distribution is a proportionality constant that relates the amount of drug in the body with the concentration in plasma. Half-life is a secondary parameter dependent on clearance and volume of distribution that describes is the time required for concentrations to decrease by one-half. Most drugs exhibit linear pharmacokinetics, where drug concentrations change proportionally with dose. For a number of drugs, where this is not the case, their pharmacokinetics are termed nonlinear. Pharmacodynamics is the scientific discipline that studies relationship between the concentration of a drug and the response or effect.