A drug is defined by U.S. law as any substance (other than a food or device) intended for use in the diagnosis, cure, relief, treatment, or prevention of disease or intended to affect the structure or function of the body. (Oral contraceptives are an example of drugs that affect the function of the body rather than a disease.) This comprehensive definition of a drug, although important for legal purposes, is rather complex for everyday use. A simpler but workable definition of a drug is any chemical or biologic substance that affects the body and its processes.
Drug administration is the giving of a drug by one of several means (routes). Drug kinetics (pharmacokinetics) describes how the body handles a drug and accounts for the processes of absorption, distribution, metabolism, and elimination.
Medications are drugs used therapeutically (that is, to treat medical conditions). Drugs often have several names. When a drug is first discovered, it is given a chemical name, which describes the atomic or molecular structure of the drug. The chemical name is usually too complex and cumbersome for general use. Next, a shorthand version of the chemical name or a code name (such as RU 486) is developed for easy reference among researchers.