As used in this discussion, substance abuse refers to excessive use of a drug in a way that is detrimental to self, society, or both. This definition includes both physical dependence and psychologic dependence. Physical dependence caused by prolonged use of a drug refers to an altered physiologic state in which withdrawal symptoms develop when the drug is discontinued. Psychologic dependence refers to a state of intense need to continue taking a drug in the absence of physical dependence. By these definitions, alcohol is a drug that can cause both physical and psychologic dependence. In this chapter, alcohol is considered to be one of several drugs of abuse. It should be remembered, however, that the extent of alcohol-related problems in the United States is so great that alcohol is often considered in a separate category from other drugs of abuse.