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 (called withdrawal symptoms) such as sweating, vomiting, and tremors which cease when the use is resumed. Also called illegal drug where its production and/or use is prohibited.
Whether a substance is legal or illegal, however, may have nothing to do with its potential for addiction or harm: alcohol and nicotine, both addictive and harmful, are legal in most countries because they generate substantial employment or government revenue through taxes.