definition for drug
drug. 1. a chemical substance that affects the processes of the mind or body. 2. any chemical compound used in the diagnosis, treatment, or prevention of disease or other abnormal condition. 3. a substance used recreationally for its effects on the central nervous system, such as a narcotic.4. to administer a drug to.rug therapy, n the use of a drug in the treatment of a patient with a specific disease or illness. drug tolerance, n the body's ability to increasingly withstand the effects of the substance being used, thereby requiring larger quantities of said substance in order to bring about the desired result.