[Early tobacco and alcohol consumption as modifying risk factors on marijuana use]

To assess the relationship between early tobacco and alcohol use and the risk of marijuana consumption among schoolchildren.

A cross-sectional study comprising data from the Fourth National Study on Drug Use in the Chilean School Population (2001). There were studied 54,001 schoolchildren aged between eight and 20 years. Predictors were self-reported tobacco and alcohol use (ever users), age at initiation of tobacco and/or alcohol use, and intensity of tobacco use. The study outcome was self-reported marijuana use (ever users versus never users) and age at initiation of marijuana use. Poisson regression and Weibull regression were used for data analysis.

Lifetime prevalence of tobacco use was high: 77%, alcohol 79%, and marijuana 23%. Tobacco consumption increased the likelihood of marijuana use (PR=10.4; 95% CI: 8.9;12.2). Later initiation of tobacco (HR=0.85; 95% CI: 0.84;0.86) and alcohol (HR=0.90; 95% CI: 0.89;0.91) decreased the risk of marijuana use. Marijuana use was higher in heavy smokers compared to light smokers (PR=3.11; 95% CI: 2.96;3.26 versus PR=1.70; 95% CI: 1.58;1.83).

Tobacco use is strongly associated with marijuana use, which is significantly associated with the age at initiation of tobacco use, intensity of tobacco use and concurrent use of alcohol. Prevention strategies should target prevention of adolescent early tobacco use.