Substance abuse refers to the improper use of medications and other illegal physically or psychologically addictive substances. This term is most commonly used to refer to excess/medically unnecessary use of prescription medications and illegal substances such as marijuana, hashish, cocaine, heroin, methamphetamine, LSD, etc. However, the definition can be expanded to include the excessive use of legal substances such as alcohol, caffeine, tobacco. We don't normally think of coffee or cigarettes as having the potential for acute abuse but it is possible to become ill from the excessive use of caffeine/nicotine. Dependence on and addiction to many of these substances requires medically supervised withdrawal.