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390 REVERSIBLE DISORDERS OF BRAIN DEVELOPMENTand may be influenced by drug prevention programs and supply reduction effortsby the government. Once exposure has occurred, however, other factors begin toplay an increasingly important role. Research over the past 30 years has demonstratedvulnerability factors that influence the likelihood of progression once exposurehas occurred. Thus it is likely that the development of an addictive disorderis influenced by multiple classes of variables as outlined in the medical model:agent (type of drug and availability), host (heredity, personality) and environment(peer group; C. O’Brien, 2005).Note in table 17-1 that the “most addicting drug,” because of the proportion ofusers who progress to dependence, is nicotine and that only 16% of those exposedto cocaine progress to dependence. The risk of addiction after exposure to heroinis 23%, whereas analgesics, which are also opiates, had only a 7.5% risk. Thismay be explained by differences in route of administration as heroin is generallyinjected, whereas analgesics are usually taken orally with slower onset of effect.Also the setting and motivation for taking analgesics is quite different from thatof heroin. Thus, risk of addiction depends on more than the pharmacological potencyof the drug, and one of the variables appears to be age of exposure.NicotineBreslau et al. (1993) reported on the role of psychiatric comorbidity in smokinginitiation. Both major depression and anxiety disorders have been associated withsmoking (Breslau, 1995). The pharmacological effects of nicotine may suppresssymptoms of these disorders; thus they would return when the smoker attempts toquit. Also, quitting is much more difficult for those smokers who began at a youngTable 17-1. Risk of addiction.DRUG CATEGORIES EVER USED (%) DEPENDENCE (%) RISK (%)Tobacco 75.6 24.1 31.9Alcohol 91.5 14.1 15.4Other Drugs 51.0 7.5 14.7Marijuana 46.3 4.2 9.1Cocaine 16.2 2.7 16.7Stimulant 15.3 1.7 11.2Anxiolytics, 12.7 1.2 9.2etc.Analgesics 9.7 0.7 7.5Psychedelics 10.6 0.5 4.9Heroin 1.5 0.4 23.1Inhalants 6.8 0.3 3.7Source: Anthony, Warner & Kessler, 1994