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394 REVERSIBLE DISORDERS OF BRAIN DEVELOPMENTPercentage of 18–19-year-olds555045Past year usePerceived risk4035302520151051975 77 79 81 83 85 87 89 91 93 95 97 99 2001 03YearFigure 17-2 Marijuana is the most commonly used illegal drug. Its use peaked in 1979among high school students and has fluctuated over the years in inverse proportion to theperception of harm. As perception of risk goes up, experimentation with marijuana goesdown.effect on motivation. Although some have shown this effect, it is difficult to extrapolatethis finding to behavior outside the laboratory (Lane, Cherek, Pietras, &Steinberg, 2005).The risk of developing marijuana addiction among users is in the range of therisk for alcoholism among those exposed to alcohol and is related to the frequencyand duration of use. Adolescents become dependent at a lower frequency andquantity of use than adults, thus fitting the pattern of adolescent vulnerability notedfor nicotine and alcohol (Chen, Kandel, & Davies, 1997).OpiatesOpiates are the fastest growing form of adolescent substance abuse. Nonmedicaluse of prescription opioids (e.g., Vicodin, Oxycontin) obtained from varioussources, including the parents’ medicine cabinet, has been growing in nationalsurveys. Nonprescription Vicodin use was reported by 10% of high school seniorsand Oxycontin by 5% in recent years (Johnston et al., 2005). Although it is probablethat adolescents are more vulnerable to the development of opiate addictionthan adults, no relevant data are available. Because opiates (derived from opium)and opioids (synthetic opiates) act on the same receptors as do endogenous opioids(endorphins), they mimic the effects of the endogenous substances. Most opiatesand opioids are not neurotoxic, although they can be taken in overdose thatcould fatally suppress respiration. Chronic opiate use has not been associated with