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 substance that affects the body and its processes.

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 (see Drug Use and Abuse: Overview of Drug Abuse).

Prescription or Nonprescription: By law, drugs are divided into two categories: prescription drugs and nonprescription drugs. Prescription drugs—those considered safe for use only under medical supervision—may be dispensed only with a prescription from a licensed professional with governmental privileges to prescribe (for example, a physician, dentist, podiatrist, nurse practitioner, physician's assistant, or veterinarian). Nonprescription drugs—those considered safe for use without medical supervision (such as aspirin)—are sold over the counter (see Over-the-Counter Drugs: Overview of Over-the-Counter Drugs). In the United States, the Food and Drug Administration (FDA) is the government agency that decides which drugs require a prescription and which may be sold over the counter.

Dietary supplements (medicinal herbs and nutraceuticals; see Medicinal Herbs and Nutraceuticals: Overview of Medicinal Herbs and Nutraceuticals are products intended to supplement the diet. These products may contain vitamins, amino acids, minerals, and herbs or other plant-derived material (botanicals). Because dietary supplements do not require FDA approval before marketing, they do not have to meet the same standards as drugs for safety and effectiveness (efficacy). Although these products are not classified as drugs, they can act in the same way as a drug does in the body and may cause health problems if not used correctly or if taken in large amounts. Because these products do not meet the FDA standards for safety and efficacy, they may not claim to treat specific medical conditions.

Drug Names: Some knowledge of drug names can help in understanding drug product labels. Every drug has at least three names—a chemical name, a generic (nonproprietary or official) name, and a trade (proprietary or brand) name (see box onTrade-Name and Generic Drugs: Overview of Generic Drugs and Drug Naming).

The chemical name describes the atomic or molecular structure of the drug. This name is usually too complex and cumbersome for general use. So an official body assigns a generic name to a drug. The generic names for drugs of a particular type (class) usually have the same ending. For example, the names of all beta-blockers, which are used to treat such disorders as high blood pressure, end in "lol."

The trade name is chosen by the pharmaceutical company that manufactures or distributes the drug. Patented drugs are usually sold under a trade name. Generic versions of trade-name drugs—manufactured after expiration of the pharmaceutical company's patent—may be sold under the generic name (for example, ibuprofen) or under the manufacturer's own trade name (for example, Advil).

Drug Groups: Understanding what group a drug belongs to is also useful. Broadly, drugs are classified by therapeutic group—that is, by what disorder or symptom they are used to treat. For example, drugs used to treat high blood pressure are called antihypertensives, and drugs used to treat nausea are called antiemetics (emesis is the medical term for vomiting). Within each therapeutic group, drugs are categorized by classes. Some classes are based on how the drugs work in the body to produce their effect. For example, diuretics, calcium channel blockers, beta-blockers, and angiotensin-converting enzyme (ACE) inhibitors are all classes of antihypertensives that work differently.

Last full review/revision May 2007 by Eva M. Vivian, PharmD