Background. Early alcohol use is associated with abuse and dependence of licit and illicit substances later in life. The role of genetic and environmental factors in this association is not conclusive.
Method. In 1992, data on substance use, abuse/dependence and psychiatric disorders were collected from 8169 male twin members of the Vietnam Era Twin Registry. The interview obtained age of onset of regular drinking (one drink/month for 6 or more months). Regression analyses of twin pairs discordant for early alcohol use tested whether the association between early drinking (before age 17) and adult substance use and abuse/dependence remained after controlling for genetic factors, family environment and covariates. Twin models tested for common genetic and/or environmental influences on early drinking and adult alcohol dependence and ever use and abuse/dependence on marijuana and other drugs.
Results. Co-twin analyses suggested the association between early regular alcohol use and adult alcohol dependence, marijuana and other drug use, and marijuana and other drug abuse/dependence could not be entirely explained by common genetic and shared family environmental factors. Genetic contributions to early regular drinking were significantly correlated with those on use of marijuana (rA=0·59), use of other drugs (rA=0·64), alcohol dependence (rA=0·54) and abuse/dependence of marijuana and other drugs (rA=0·63 and 0·66). Small but significant unique environmental correlations (rE range 0·11–0·22) indicated that familial factors could not entirely explain the association between early alcohol use and later substance use, abuse and dependence.
Conclusions. Early regular drinking is associated with later alcohol dependence and use, abuse/dependence on drugs. The association is not entirely explained by genetic or shared family environmental factors. This suggests unique environmental factors contribute to transitions from early regular alcohol drinking to use, abuse and dependence on alcohol and other substances.