Vulnerability to substance abuse has general genetic, familial, and nonfamilial environmental factors, as well as factors that appear to be specific to a particular class of substances. A family history of substance abuse is one of the strongest risk factors for development of a SUD (Merikangas et al., 1998). Studies have demonstrated that there are genetic influences on the risk for substance abuse (Tsuang et al., 1996) and that, at least among men, abusing one category of drug is associated with a marked increase in the probability of abusing other classes of drugs (Tsuang et al., 1998). One of the strongest predictors for presence of an SUD is the presence of another SUD (Bierut et al., 1998).
Much of the evidence for the heritability of the general and specific vulnerability for SUD is taken from studies of familial aggregation. Bierut and colleagues (1998) compared siblings of probands with alcohol dependence and those of a control group for the presence of lifetime SUDs. Siblings of alcoholic probands were not only more likely to have a lifetime alcohol use disorder, but they also had an increased risk of cannabis, cocaine, and nicotine dependence. Fifty percent of the alcohol-dependent siblings of alcohol-dependent probands had an additional diagnosis of cannabis and/or cocaine dependence. What is compelling with respect to understanding the risk for multiple substance dependence is that the siblings of cannabis-dependent probands had an increased risk of cannabis dependence, siblings of cocaine-dependent probands had an increased risk for cocaine dependence, and siblings of habitual smokers were at higher risk for nicotine dependence (Bierut et al., 1998). In another study, Tsuang and colleagues (1998) demonstrated that there is a general drug abuse vulnerability factor with genetic, family, and nonfamily environmental components that is shared across all drugs of abuse, in addition to genetic factors that appear to be unique for most classes of drug abuse. So although there appear to be nongenetic general and specific factors for familial transmission of vulnerability to SUDs, multiple SUDs among probands render increased vulnerability to multiple SUDs in relatives, at least through both drug-specific and common genetic factors.
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