Genetic Influences in Drug Abuse

Individuals are differentially vulnerable to substance abuse. Everyone has access to addictive substances. Not everyone who has an opportunity to use an addictive substance does so, and not everyone who uses an addictive substance becomes addicted. Sixty-three percent of individuals 12 years of age or older report never using illicit drugs or psychotherapeutics, 29% never use cigarettes, and 17% never use alcohol (1992 National Household Survey; Substance Abuse and Mental Health Services Administration, 1993). Sixty-five percent of individuals reporting access to marijuana use the substance, while only 16% of those having access to heroin report use (National Institute on Drug Abuse, 1991).

The likelihood of continuing drug use also varies from individual to individual. Illicit drug use often begins in early teen years, peaks in the late teens and early twenties, and can decline substantially thereafter ( see ref. 39). However, some individuals continue to use drugs into later adulthood. Ninety-three percent of individuals who used alcohol, 60% of cigarette smokers, 19% of heroin users, and 8% of hallucinogen users continued to use drugs at the end of their third decades of life (see ref. 57).

The frequency and consequences of illicit drug use, factors that underlie most definitions of drug addiction, also vary from user to user. Forty-five percent of individuals using marijuana report using it 12 times or more; half of these report using it once a week or more (National Institute on Drug Abuse, 1992). Variability can also be seen in reports of symptoms of dependence, even among regular users. Forty-two percent of regular cocaine users did not report any symptom of cocaine dependence (National Institute on Drug Abuse, 1991).

Observations such as these suggest that genetic and environmental conditions that differentially predispose individuals to drug-taking behavior and to the transition from drug-taking behavior to established and maintained drug abuse might be found.