A simple and workable definition of a drug is any chemical or biologic substance that affects the body and its processes. U.S. law defines a drug as a substance 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.Some people incorrectly use the word drug to mean only a substance that produces a pleasurable sensation. Drug abuse—the excessive and persistent use of mind-altering substances without medical need—has accompanied the appropriate medical use of drugs throughout recorded history. Some drugs with potential for abuse have legitimate medical purposes, and others do not.