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Substance Abuse and Addiction 389Estimated probability of first drug use (%)%302724211815129630Marijuana0 5 10 15 20 25AgeAlcoholCocaine30 35 40 45 50Figure 17-1 The vast majority of substance abusers begin their drug use before the ageof 25 and most prior to age 21. (Wagner & Anthony, 2002).study, initiated in 1975, has charted annually the extent of drug exposure in 8th,10th and 12th graders in the United States using similar methodology (L. Johnston,1996). This series of studies tracks the changes in drug usage over the years, andthe prevalence of experimentation with various substances has remained consistentlyhigh among students since the studies began. The epidemiological datasuggest that there may be factors in adolescents that encourage the initiation ofsubstance abuse. Of course risk-taking behavior is a well-known characteristic ofadolescence (Steinberg, 2004).A neurodevelopmental perspective has been proposed by Chambers et al.(2003), who reviewed data from animal and human studies suggesting that adolescenceis a period of change in brain organization and function. These changesare structural, involving synaptic pruning, and functional, with relative increasesin promotivational dopamine function and relative reduction in inhibitory serotoninsystems. The result tips the balance toward impulsivity and risk taking.Drugs of abuse, which are excitatory to dopamine systems, may be more effectiveduring this period. Thus adolescents are more likely to take the risk of experimentationand, once exposed, are more susceptible to the seductive effectsof these drugs.Although exposure through drug experimentation is high, most of the adolescentsexposed to drugs do not progress to abuse or addiction. Table 17-1 showsthe proportion of American adults who were exposed and the percentage of thesewho progressed to addiction (Anthony, Warner, & Kessler, 1994). The first requirementfor the development of substance abuse or addiction is drug exposure,taking the first dose. This depends on availability and willingness to take risks