In 1938, the Federal Food, Drug, and Cosmetic Act was passed by Congress. This Act, and later amendments, established the current system of drug regulation in the United States. It set up classes of drugs and the requirements for approval of these drugs.

The two main classes of drugs are: (1) non-prescription drugs, and (2) prescription drugs. The non-prescription drugs are commonly called over-the-counter, or OTC drugs, and can be bought without a prescription.

Prescription drugs (or legend drugs) are drugs that require a prescription because they are considered to be potentially harmful if not used under the supervision of a licensed health care practitioner. Certain prescription drugs have additional controls placed upon them. These drugs are called controlled (or scheduled) drugs.