An increase in the prevalence of cannabis use and admissions for treatment of cannabis-related problems again raises the question of whether early cannabis use increases the risk of later substance abuse and dependence. Later-life substance abuse may be due to environmental or genetic factors, or it may be related more specifically to early, “gateway” drug exposure. Lynskey and colleagues compared drug use and dependence in monozygotic and dizygotic same-sex twin pairs discordant for early cannabis use. If genetic or environmental factors are predominant in the development of drug-related problems, both twins, regardless of early cannabis use, would be equally at risk for substance abuse and dependence in later years.

Telephone interviews were conducted with both members of 2,765 twin pairs and one member of an additional 735 twin pairs. Of the total sample, 861 members reported initiating cannabis use before the age of 17 years. Of these, 311 (36.1 percent) were twins from pairs in which their co-twin had not used cannabis by age 17. The study thus had a subset of 622 same-sex twins discordant for early cannabis use. Participants were questioned about their lifetime drug use and dependence; other risk factors associated with cannabis dependence (psychiatric disorders, early tobacco use, and early regular alcohol use) were included in the study as control variables.

The authors found that relative to the co-twins who hadn't used cannabis before the age of 17, those who had used cannabis had higher lifetime rates of other drug use, drug dependence, and alcohol dependence. The unadjusted conditional odds ratio was 2.1 to 5.2 times higher in earlier users than in their co-twins; in all areas except sedative abuse/ dependence, the findings were statistically significant. Controlling for known risk factors of later drug abuse and dependence had only negligible effects.

The authors conclude that early initiation of cannabis use is associated with a significantly higher risk for other drug use, abuse, or dependence. Mechanisms that may pave the way for later abuse include pleasure and perceived safety of early experiences or access to a wider range of drugs through drug dealers. Although the study findings were consistent with the conclusion that early cannabis use is a risk factor for any drug use, abuse, or dependence, the authors stress that the associations found in this study do not establish a causal role of early cannabis use with drug use progression.