Alcohol and nicotine are the most commonly misused substances in the United States. Nearly 20 million Americans are alcohol abusing or dependent, and almost 50 million Americans smoke cigarettes (Substance Abuse and Mental Health Services Administration (SAMHSA), 2010). Alcohol dependence and nicotine dependence frequently co-occur in the same individuals. Furthermore, nicotine dependent individuals are four times more likely than the general population to be alcohol dependent, and people who drink are three times more likely than the general population to smoke (Grant et al., 2004). Identical twins are twice as likely as fraternal twins to become alcohol and nicotine dependent if the other twin is dependent (Carmelli et al., 1993; Swan et al., 1997). Alcohol and nicotine may enhance motivation to use either drug by activating common brain targets that are responsible for their reinforcing effects. They may also exert synergistic effects on behaviors which may contribute to their concurrent use.
