Adulteration, the lowering of the quality of products, especially foods and drugs. Adulteration is accomplished by (1) using harmful, tainted, unsafe, impure, or inferior ingredients; or (2) altering or deceptively treating the products in order to give a false appearance or to hide defects. There are federal and state laws, as well as municipal ordinances, that forbid or restrict the shipment and sale of adulterated products. Federal law regulates the safety of food additives.

The adding of water to milk is an example of lowering quality by using a cheaper ingredient that in itself is not harmful. Some food preservatives, such as sodium benzoate, are safe in small amounts, but can be injurious to health—and are thus considered adulterants—if excessive amounts are added to food preparations. Similarly, many additives that are intended to prevent spoilage of foods, or improve their taste and appearance, are considered adulterants if used in amounts greater than those specified by law or regulation.

Drugs are considered adulterated if the strength, quality, or purity of any of the active ingredients fails to meet the specifications listed on the label. In order to control adulteration, the federal Food and Drug Administration inspects drug manufacturing plants at regular intervals and makes frequent tests on samples from drug shipments. Furthermore, drug manufacturers are required by law to indicate on the container the contents of each product.