Early smoking onset linked to higher risk of cannabis use

Young people who start smoking at an early age have a much higher risk of starting to use cannabis by the time they turn 17. Risk factors that increase the likelihood of starting to smoke include externalising problem behaviours such as impulsiveness. Smoking, in turn, is a significant risk factor for cannabis use. These results have been confirmed in a project researching smoking at an early age and externalising behaviour as predictors of drug use. The project has been funded by the Academy of Finland's Research Programme on Substance Use and Addictions.

By the time the adolescents were 17, some 15 per cent of the girls and 12 per cent of the boys participating in the study had used cannabis or other illicit substances at least once. The identified predictors of drug use were female gender, binge drinking, father's binge drinking, smoking peers and acquaintances with drug experience, and aggressive behaviour among boys. Early smoking onset was an especially powerful predictor. Compared to never-smokers, those who had started smoking by the age of 12 or earlier were 26 times more likely to start using drugs by age 17 (Korhonen et al. 2008).

The project made good use of data collected in the FinnTwin12 study, a Finnish-American birth cohort study initiated in 1994 to examine genetic and environmental determinants of precursors of health-related behaviours. The project gathered questionnaire and interview data on twins aged 12, 14 and 17, and also collected information from their parents and teachers. The present study also involved researchers from the Netherlands in connection with the Vidi-project and the TRacking Adolescents' Individual Lives Survey (TRAILS) project.

Gateway theory revisited

In one of the studies of the project, researchers examined individual, peer and family factors as predictors of starting to use drugs at the age of 17, the age by which some 15 per cent of the girls and 12 per cent of the boys participating in the study had used cannabis or other illicit substances at least once. Early smoking was identified as an especially significant predictor. This finding was replicated also in the data collected in the Netherlands (Korhonen et al. 2010a).

"The findings support the gateway hypothesis (Kandel et al. 2006), which asserts that licit substances such as tobacco and alcohol are a stepping stone to harder, illicit drugs. The theory has come under much criticism in recent years. Many have argued that certain common factors may explain both smoking and drug use, factors such as problem behaviour and genetic influences. That's why we wanted to look into these issues more deeply in our studies," says Adjunct Professor Tellervo Korhonen, one of the researchers on the team.

The second sub-study compared alternative structural equation models. The first model proposed a causal association between early-onset smoking and the subsequent onset of drug use, while the second model proposed that both smoking and drug use are a result of common genetic and/or environmental vulnerabilities. The first model, where the early smoking affected the early onset of drug use, had a somewhat better fit with the data. However, the researchers were not able to rule out the role of common genetic factors either (Huizink et al. 2010). A similar conclusion was reached in a study based on US material, published shortly after this study (Agrawal et al. 2010).

Feeling the need for weed
 
The third sub-study analysed the associations between externalising problem behaviour, the early onset of smoking and drug use. The results suggest that externalising behaviours, such as hyperactivity, impulsiveness and aggressiveness, are predictors of early smoking, which in turn is a strong risk factor for drug use. In most of the models analysed, the association of problem behaviour with drug use was mediated through early smoking (Korhonen et al. 2010b).

"This can be interpreted so that impulsive tendencies lead to an increased risk of experimenting with different problem behaviours. If young people have access to tobacco, it's very likely that they'll try it quite early, which may later lead to experimenting with other psychoactive substances such as cannabis," says Korhonen.

The study was part of a larger research project led by Professor Jaakko Kaprio, "Predictors, neuropsychological correlates, and consequences of cannabis and alcohol use among Finnish young adults - a twin and population approach", which was funded by the Academy of Finland's Research Programme on Substance Use and Addictions.