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Substance abuse tends to be a chronic, progressive disease. The first or initiation stage—from nonuser to user—is such a common feature of becoming an American adult that many authorities call it normative behavior. At this stage, substance use is typically limited to experimentation with tobacco or alcohol (so-called gateway substances). During adolescence, young people are expected to establish an independent, autonomous identity. They try out a variety of behaviors within the safety of families and peer groups. This process often involves experimentation with psychoactive substances, usually in culturally acceptable settings. Progression to the second or continuation stage of substance abuse is a nonnormative risk behavior with the potential to compromise adolescent development. The American Psychiatric Association has outlined criteria to judge the severity of substance use that progresses beyond the experimentation stage to substance abuse or dependency. Progression within a class of substances (eg, from beer to liquor) and progression across classes of substances (eg, from alcohol to heroin) are the third and fourth stages of substance abuse. Individuals at these stages are polysubstance abusers, and most manifest one or more symptoms of dependency, such as tolerance or withdrawal. The transition from one stage to the next is often a cyclic process of regression, cessation, and relapse. Common physiologic effects and symptoms of intoxication (which can occur at any stage) and withdrawal (a symptom of dependency) for the major classes of substances are shown in Tables 5–1 and 55–2.

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Table 5–1.Physiologic effects of commonly abused mood-altering substances by organ/system.

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