The definition of drugs can be classified as the following: (1) A natural or synthetic substance which when taken into a living body affects its functioning or structure, and is used in the diagnosis, mitigation, treatment, or prevention of a disease or relief of discomfort. It is also called legal drug or medicine. A legal or medicinal drug (such as amphetamines), however, can be harmful and addictive if misused.
(2) A habit forming stimulant or narcotic substance such as alcohol, cannabis, nicotine, or a derivative of cocoa or poppy which produces a state of arousal, contentment, or euphoria. Continued or excessive use—called drug abuse or substance abuse—of such substances causes addiction or dependence. Thereafter any attempt to discontinue their use results in specific reactions such as withdrawal symptoms, sweating, vomiting, and tremors which cease when the use is resumed. It can also be called an illegal drug in which case its production and/or use are prohibited. Whether a substance is legal or illegal has nothing to do with its potential for addiction or harm. For example, alcohol and nicotine are both potentially addictive and harmful and are legal in most countries.